yes, if you are talking about piety. the problem is, piety was gained not through training a skill, but through extra-skill system (rp, in this case). just this fact, can make it unbalanced. why do some chars have all their strength in terms of skill and how long they train it, and some others (only some others, not all the classes) get stronger with something that has nothing to do with skill? should we then, add rp to the list of skills like herding and provocation and anatomy?since it can be "gained"? but then, why only a few classes get to have it?piety can not be qualified as a normal skill because it's not "trainable" like any other skill, it's a totally different thing.
Anyways, I found priest easy to train compared to how strong it is (except for res spells, never actually trained it), but meditation and healing were surprisingly easy considering how good they are.
poisoning is a LOT harder than those two combined times a lot. look how many heightened priests there are, and how many assassins have more than 90 poisoning, not even saying 100.
i'm not sure if the hardest classes to train are the best classes like monad suggested. i don't know how hard or easy it is to train a barbarian's str, but apart from that the class seems very strong and not hard to train. don't see anything hard except for poisoning for an archer (and archery which consumes materials), and 85 poisoning is doable. meditation and healing for priest were easy (atleast for me).
assassin (not counting poisoning, which is omgsohard to train) has other hard skills to train like hiding and stealth, and they don't seem to make your char better than a 100 med/100 healing priest
i guess different skills give different advantages, but i would say some skills are definately harder, but not necessarily better as in rewarding you more.
peacemaking is the easiest skill to gm, atleast from all the skills i've trained, and it can be very useful in some hunting situations.