Ok, so, I do think there's some things that could be changed about Legacies, but I think that they are overall reasonably well balanced at this point, with some outliers.
I especially can't understand the 'Nomadic legacy is useless without Nomadic Government' point of view, which... yeah no, it gives powerful bonuses even if you aren't the prefered government type, much more than say Bureaucratic.
So, here's my evaluation of all the DLC specific Legacy tracks, I might do the Base Game ones later.
Adventure:
This one is alright. It mostly seems to be themed around the memetic Haestein playthrough with Varangian Adventures all over the map. It also has some great generally useful bonuses, like halving the cost of Send Gift, which is great, and the bonus army movement speed and supply capacity are great especially for raiding tribals/nomads. I think it's fine to leave it like this.
Pillage:
This one is really good, especially for raiding and if you have access to great Heavy Infantry units (like the Norse do). But even after you lose the ability to raid, it's still very good for Sieges and gives a variety of bonuses that are always good, like increased chance to capture valuable characters and ransom them for more gold and prestige. Once again, I wouldn't really make any changes.
Metropolitan:
This one is all about Cities and you want many cities to take advantage of this. I think it works best if you are Administrative, since they can hold cities without penalties. That said, it's frontloaded, with +20% County Development per City in said County being very powerful and the Town Maven traits providing all your dynasty members with bonus Stewardship and Learning. Overall, I don't think it needs to be changed significantly, maybe buff Tier 4 a bit, since it's kinda underwhelming.
Coterie:
I think this is the worst Dynasty Legacy in the entire game. It only really works for the Clan Government, since Feudal and Administrative especially are going to create Cadet Branches of your Dynasty as soon possible. They can't do that if you are a Clan Government, in which case this Legacy track becomes more useful.
That said, the bonuses aren't even that good for its niche and I feel like it can use a buff. For example, being able to disinherit your kids without a Renown penalty isn't that great, since it also reduces prestige and gives you a large Opinion debuff from your vassals.
There's also the one ability to call other House members into their wars even if they are not the House Head. The player is almost always gonna be the House Head, so this is mostly for the AI. But I don't think it's that good for them either.
Overall, it's best to think of this as the Legacy track for playing as the Clan Government.
For possible changes for Coterie, I guess I'd have Tier 1 also increase Aptitude for all Court Positions for House Members by 10, Have Tier 2 effect moved to Dynasty and doubled to 20% if they are from your House, have Tier 4 also boost the speed of the Find Secrets spymaster task and have Tier 5 also increase acceptance chance for revoking Titles from House Members and give +10 House opinion.
Brilliance:
The Dynasty Legacy for playing as a Persian culture with the Clan Government. The most narrow in application, but some of its bonuses also work regardless of Government Type or Culture. Embellish Capital is costly, but is great if you play Tall. You can also combine this Legacy track with Glory to get super fast Personal Schemes. I think it's fine, great when you play in Persia and has enough generally applicable bonuses that it's still relevant even if you change government type and culture.
Bureaucracy:
Bureaucracy has less generally applicable bonuses than Brilliance and you need to be Administrative to take advantage of it. That said, Tier 5 also reduces Claimant factions, which is strong and generally applicable. You also get +0.1 Monthly Development for all of your Dynasty members and some Popular Opinion. And all the Administrative-specific bonuses are great. I think this one is fine as it is too.
Nomadic:
+10 Peace Acceptance is very strong, stacks with Peacemaker from the Gallant tree into being able to make the enemy surrender at 80% War Score. +1 to size of Men-at-Arms is also great and this is all Tier 3. This also gets many bonuses to Horse Archers, allows you to recruit a squad of Keshiqs, which are like a much better version of Heavy Horse Archers and bonuses to tributaries (everyone can have Tributaries). It also gives some nomadic-specific bonuses and raiding bonuses which are also strong.
In fact, the amount of bonuses that won't be applicable to non-nomadic rulers at all is on the lower end. I think that if anything, this legacy Track might be somewhat overpowered. About the only change I can think of is to move the Peace Acceptance to Tier 5 instead of Tier 3. I don't think it's overwhelmingly strong, but it has a lot of great stuff.
Customs:
A lot of Learn Language schemes bonuses, but it also makes Languages give bonus Cultural Acceptance, so it also makes them much more useful, among all the other Cultural Acceptance bonuses this track gives you. The Cultural Fascination Progress bonus is also great and the Celebrate Other Cultures decision is very strong and greatly boosts Development and Gold gain. Overall, this is the legacy track for when you are ruling a huge empire with many different cultures. No real changes needed.
Activities:
Greatly increased bonus XP to a number of different Leveled Traits like Traveler and substantial bonuses to Grand Activities and Pilgrimages. The highlights are Grand Wedding bonus acceptance and increased possibility of getting a Strong Hook and also Tier 5 for having the first Grand Activity a House Head hosts during their life free.
Those cheaper Grand Weddings also makes your AI Dynasty members host them more, which you can then attend for Stress loss and various bonuses like lifestyle XP. Overall, this one is great, especially Tier 5 free Grand Activity.
Heroic Bloodline:
Legitimizing Legend Seeds are the hardest to come by, so a free one once per game is good, that said, I'd give some small permanent bonus to it as well, like -10% Legend Upkeep Cost. Ancestral Claims depend on the legend itself, but some unique ones could end up with your dynasty having ancestral claims to half of Europe, so it's much better than you'd think. And if you are part of a huge empire, this is the best way for your dynasty to get a claim on that empire's title.
Aside from that, this legacy track also gives some traits only for the character that finishes the legend of a specific type (and they might already be at the end of their life by the time you finish this legend) and lower Legend Creation Cost (which is a drop in the bucket compared to the real gold sink, legend upkeep cost).
Tier 5 allows you to build a Legendary Watchtower building when you finish a Legend and that one basically boosts defensiveness of the county it's built in in a major way as well as giving a good boost to Man-At-Arms stationed there. If you play tall, this can be worth it.
Overall, I think this one is decent. Maybe change the Legend Creation Cost price decrease to lower Legend Upkeep and give some small permanent bonus to Tier 1.
Legitimacy:
I think this one is underrated by people, but also that it needs some buffs.
Tier 1 is fine, some Popular Opinion, but +0.1 Control Growth is good, since AI is worse at managing Control than players. Especially good if you combine it with the Law legacy for +0.3 Control Growth for all Dynasty members.
Tier 2 is... yeah, +10% Legitimacy gain all the time would be alright maybe, but this one is limited to only the first 10 years in power. I think this one needs to just increase Legitimacy gain by 10% all the time and be doubled to 20% for the first 10 years in power.
Tier 3 unlocks the Espouse Legitimacy intent, which is a great way to farm Legitimacy.
Tier 4 reduces Claimant and Dissolution Factions against you, which is awesome for realm stability, especially combined with Tier 5 Bureaucracy and Tier 4 Glory.
Tier 5 allows you to have Level 5 education of the chosen type. Choose either Stewardship or Martial, they are the best ones. I think this one should also boost the Education Score of Dynasty Members, to make it more likely that Dynasty Member actually get that education or give a 50% chance for Dynasty Members who reach Level 4 education of the chosen type to get Level 5 instead. And maybe also boost Tutor Aptitude by 10.
Another thing is that for Administrative realms, the Tier 5 of this legacy track is seriously overshadowed by the Education Hall internal mansion upgrade, which unlocks all Tier 5 education traits and also boosts Tutor aptitude.