I think the treatment of both is quite fair, although we need to remember that the modern depictions of these historical figures are reactions against older tropes and shouldn't be looked at in isolation. If we remember that yes, Genghis Khan did ride across the known world slaughtering hundreds of thousands of people and destroying entire cities, talking about his multiconfessional state or his prowess as a leader and administrator is quite fair too since it helps to stop us from only thinking of these people as monstrous gargoyles or cackling orcs or something. The Mongols were not a pack of savages or some kind of apocalyptic invasion force from beyond the known world, they were real ordinary people living according to a way of life that was really quite normal for many thousands of people across Eurasia, and noting how competent and efficient they were reminds us of that fact. If anything talking about the good points of the Mongol Empire helps return their agency to them and lets us really judge them for choosing to pillage and kill on such a scale.
Attila the Hun on the other hand has a different problem in that he has been a ridiculous trivialised figure for many decades. The Mongols at least get exposure from having a sovereign state of their own and documentaries and films about the life of Genghis Khan are pretty popular. Attila on the other hand was never treated seriously by popular media until games like Total War: Attila came on the scene. Before Total War, the average member of the public (outside Hungary or Turkey of course) knew virtually nothing about Attila, and what did exist was essentially a caricature of a generic barbarian leader: loads of women, brutal, probably half-naked most of the time, drunk from skull cups etc. There was a famous book about using Attila's strategy as an inspiration for good management techniques, but even that's just a novelty thing. And if you asked who the Huns were it was more likely that you'd get an answer related to the Kaiser than anything else. I'm sure the actual details about Attila remain mostly obscure to ordinary people today, but at least games like the recent iteration of the Total War series portray him as a serious force of nature, something to be legitimately frightened of. That might be the kind of portrayal that the Mongols are trying to get away from, but at least its better than "generic warrior dude from an Animaniacs cartoon".
Not in the slightest. Horse Lords is marginally better because at least it personifies the nomads, but both of them are guilty of turning steppe nomads into agents of mass destruction. If you made a game about the Americas in which the natives were little more than armed robbers only interested in scalping and abducting white women there would be an uproar. If you made a game about colonisation in which the Europeans were brutal plunderers only concerned with stealing gold and taking slaves you'd probably be accused of creating propaganda. But for some reason stereotyping nomads as destroyers of civilisation who can't even govern themselves and just leave wastelands in their wake is fine. Yes, the Mongols did do this, but their campaigns of extermination were really quite unusual and yet another way in which they were the exception. Other nomads did engage in raids (the most extreme example being the Russian Tatars), sometimes migrated into territory claimed by others and their considerably military prowess meant that they usually ended up interfering with the internal politics of neighbouring sedentary states, but they didn't do all of this with the aim of "sacking" all the settled civilisation of conquered lands down to nothing and populating it with sheep and cattle.
The Mongols, given that they had an eight hundred year advantage in technology and were commanded by a bunch of military geniuses. And I'm not certain that by Attila's time the Huns still fought in their original nomadic style anyway. The Hunnic "Empire" was really more of an eastern European tribal confederation of various Germanic and otherwise Indo-European peoples cemented around a Hunnic elite. Hun armies were full of Germanic warriors, and even Attila himself had a Germanic name.
