They are useful, but we're talking lategame here. Very lategame. Like when you have too much industry, most tech unlocked, and enough xp to play around with.
A 2w modern tank with a heavy howitzer and Henschel as soft attack boosting MIO gets me 68.7 soft attack for 37.36 IC (for this particular tank model I built). That is the baseline against which you'd have to measure potential SPA models. The SPA I built here, at 3w, delivers 113.5 soft attack for 30.44 IC, also Henschel.
If we look at it from a pure per-width standpoint, you'll notice that 37.833 (SPA) are bigger than the tank's 34.35, and the SPA is also cheaper to make.
To fill 6 width, you'd need 150 tanks, but only 100 SPA, so you get slightly more soft attack per width considerably cheaper. And if you don't use howitzer on your tanks, but any cannon for the hard attack? In singleplayer not that important, but if you for some reason have to use cannons on your tanks, the SPA is a good solution. At least if you want armor. Art3 with full soft attack Rheinmetall will pull 71.2 soft attack on 3w, uses 36 pieces of equipment for roughly 5 IC, so the entire unit is cheaper than six pieces of lategame tanks/SPA.
SPAA is in a tighter spot, as it uses 2w, while normal towed AA uses only 1w. My lategame SPAA brings 68.0 AirAttack for 2w, the towed AA3 offers a good 52.4 for 1w. If you don't care for division armor, the towed AA will win and it's not even close. The opportunity with SPAA lies in adding the other stuff, not just armor, but also a bit of breakthrough, a bit of extra fuel, dozer blades etc.
It's also worth a discussion if you should put AA into the unit lines at all. And I'd generally agree, it has a rather comfy spot in the support line, despite lategame only having 44.3 AirA for using only 20 pieces of equipment. Still enough to do your thing, no need to change that. But lategame? Flametanks, Logistics, SPHH, armored engineers, heli recon, armored signal... there are lots of good things to slap into those slots, so it might be worth moving AA into the SPAA slot, increase your armor/stats while at it, get more AirA out of the deal, and use the flat +x% bonuses that the other support companies provide.
Not exactly economic with your IC, but oh well, it sparks joy when I look at it, so I don't throw it away!
Anyway, it really depends on what MIOs you have or don't have. If you don't have a good selection, like Germany certainly has, then those hyper-specialized high stat equipment variants will probably not be viable. That's the grain of salt I'd throw in here.