Ars Magica is in HEAVY development still. Of course a lot of parts feel off and slowly over time that changes one update at a time. This, for those that had not caught the fact, is also including blatant intent towards compatibility with Thaumcraft. Both Mithion and Azanor working from the same source material (AM has outright permission to use names, etc.).
As for depth, in addition to nearly being REQUIRED to explore the world a lot in order to find all the spells without it costing you extreme amounts of resources (a spell research table primarily), the spell system is remarkable by itself but the second you look at spell creation...well... This is a rough calculation of 70 spells, 3 modes per spell, and up to 5 spell layers during custom creation (assuming friends are stabilizing for you) will provide a rather insane
70^15 in potential combinations! Granted, some spells have similar traits and some may not do anything tangible, plus new spells are likely to be added at times so it will always be a rough measure. If you have no clue what the numbers are, pull up your PC-provided calculator...
Of course, to each their own in how they want to play. Personally, I will mix Ultimate with Ars Magica and a few others. I'm a somewhat meandering player, so it is hard for something to really be OP to my taste and if it is, I just stop using it when it feels less fun.
Give Ars Magica some time to develop, and importantly, refine and polish.