It's less of an opportunity cost if you just go to like -50% from a couple of monuments and then upgrade the 50% cost advisors you get via events.
So, yes, advisors and disinheriting is way overpowered compared to the original design of the game. You would get these 1-2-0 kings that lived for 40-50 years and it would affect your gameplay, albeit to a smaller degree if you were big enough to afford +3 advisors (this would typically ruin your economy in the early and mid game). These days, you can get up to +5 advisors (on their own, they're the equivalent of a 4-4-4 monarch with level 1 advisors, which is a solid state to be in anywhere), plus you never end up with garbage tier rulers if you are playing monarchies.
In my opinion, it's less about is this OP or not. The more important question is: what game design philosophy is a more enjoyable experience? Is it a 'better' problem to have more mana than you can use, or falling behind in tech/expanding because you have a mana deficit? It's more a matter of preference, I think, and with the way the new institutions are designed you might be falling behind even in Europe with decently statted monarchs.
I think the issue is that there are no 'meaningful' ways of investing mana. Mil overflow? Spam generals for professionalism. Admin overflow? Stab +3 and then develop (which does nothing). Dip overflow? Gold mine upgrades then develop (which again, in most expansionist games feels pretty useless). I'd like to weigh my options. What if I could expand my governing cap by spending a combination of dip and admin and time, instead of government reform? What if I could turn the red X into a green V for 'Offer Vassalization' by sending a diplomat and investing dip points?
I don't like all these 'instant click and a thing happens' effects. Let me send a diplomat to my colonies and it will drain 3 mana and 3 admin for 5 years (180 of each), and after it is completed the colony gains a permanent goods produced bonus and a liberty desire reduction. Like... let me grow in other ways than just conquering anything that's bordering me and waiting for AE to tick down.
