Right now Empire Maintenance (hereafter called corruption for ease of typing )rises very quickly in DEI, at least if you go for a more aggressive campaign. You can easily be at the 100%s of penalty by what could be considered middle game.

Now, there are ways to counter it: Buildings, techs, and skills on both governors and, here's the important part, generals/admirals/statesmen.

Buildigs are nearly irrelevant: The best it can get you is around 1%, maybe 2% reduction in the penalty, but there's also a number of quite important/useful later game buildings that increase the penalty, so at the very best you're breaking even there.

Techs don't give in total that much reduction in corruption, and they generally come rather late specially if you don't "turtle".

Governors have great skills there, but they are limited in number, and take rather long to go up the ranks.

Now, generals? Between both the bureaucrat and sovereign skills, there are skills that reduce the penalty. Political Reformer for the bureacrat, giving -1%/-2%/-3% corruption and +4%/+8%/+12% research speed. For the Sovereign, there's Unwavering Patriot, giving -1%/-2%/-3% corruption. The latter also gives out cultural convertion (local province), and both of them give public order (local province) at higher levels, but these are irrelevant for our purposes here.

So, at first glance, it makes perfect sense to have all your generals to get the bureacrat line, and then the sovereign line if they can afford it, because not only it can reduce corruption, the research bonuses also can pile up quite quickly (and considerably faster than libraries at that too).

A lot of level ups? Even without battles, a garrisoned general goes from level 1 to 2 in a small handful of turns, and to 3 in a little bit more. And if you have the military academy line, it already starts at higher levels, and if you garrison then in the city that has it, he gains XP even faster.

So far, it looks quite powerful, yes, but not brokenly so. If you focus your generals such, instead of your armies being led by superhuman warriors and leaders they are led by superhuman bureacrat and politicians who also command soldiers on the side.

The thing is, since these are faction wide bonuses, as far as I understand they still apply even if they are statesmen, meaning that, for a few thousand gold you can recruit a general, get him a few levels (or have him already be recruited with them if you have the academy), make them reformers and patriots, send them back to politics and repeat until your corruption is more manageable. And as a sidebenefit, you'll be researching stuff at a ludicrous speed. In my current game, at 145 turns in, I'm researching at 300% speed given all the bonuses from governors and generals I have.

Now, it's not really feasible to simply remove these, because without them, corruption becomes pretty much unmaneagable in the late game, at least in more aggressive campaigns. Nonetheless, a deeper look into it is deserved, if nothing else to make so that having armies and armies of reforming patriots not be a game winning strategy for those who plan to expand a bit faster.

Preliminary speaking what I could see a way to handle these would be to reduce corruption buildup, remove the research and corruption reduction from the general skill trees, and increase the research and corruption reduction from buildings and techs. Governors should be fine as they are, given that they don't have the numbers to break the game anyway, but buildings feel too weak for how limited they are, and statesmen too strong for how you can always get more of them, while if you don't use them as such, corruption breaks your kneecaps...