OK, just finished my third playthrough (the first two were nearly a year ago). It is also the first where I had the N'Mar, so in some parts I did not know what to expect. Since the impression are wuite fresh, here are they. My playthrough was in "hard". When I reached Fost, the party was lvl 15-17 for "A-Team", and 13-14 for the "B-Team".
I seldom used "generic" abilities (fighter, thief, mage), instead I put most upgrades into "special" abilities. If a mage had a few more SP made a lot less difference than having access to lvl 3 fireball. And SP/HP upgrades were still more useful than the rest on the generic page.
I found the Chamby and Draco to be a bit too strong. I only used both of them in the hardest fights, anything else felt to "cheaty". Any one of them could decimate the ranks quite fast. Myrth was not nearly as useful as damagedealer, but was able to heal the whole group at once. For Apolimesho I never found a useful role, the enemies died fast enough even without buffing. I felt the slot was better used by someone who really dealt damage when it came to it.
Thieves were very useful too - I usually had Elenor-thief and Karen in the front-row. They dealt damage, had useful special attacks, AND had the option to throw bombs. Bombs were scarce, which is a good thing, otherwise thieves would be overpowered as well. The tanky qualities of fighters were what I never needed. Even on hard Thieves with medium HP plus Healers (Elenor, Myrth, Potions) were enough. Loren caused a lot of damage too, but the attacks of Ramas and the other fighter were plain underwhelming. Maybe except Trouble, when against fire weakness.
Three fights were hard for me: the vampires in castle N'mar, the demons just before entering Fost's stronghold, and training against Breza. The training should be hard, of course. The other two fights were hard because I tended to spend the Spellpoints in the first battle, and have nothing left in the following ones. Also the three drakes with Inferno III were just plain evil. Please add more like it. Well, maybe I should add Grob, who made me use up some potions.
Some of the sidequests were too easy: outnumbered foes who were two levels below the minimum level the party could have reached at the point at all.
As I wrote above, buffing never felt useful. The same goes for attacks which only gave the enemy debuffs. Nice when a sideeffect of a high-damage attack, but useless on their own.
When it came to choosing the team, I mostly went with whoever I liked (as character). Maybe it is lucky this gave me a strong team, or maybe just suited my playstyle or the chars are balanced anyway. In any case "hard" difficulty seldom felt really hard, without me putting too much thought into finding the "best" upgrades, builds, ...
I notice other people had a bit different experiences. Which may very well be because of other builts and play styles. I hope this may help nevertheless when designing the next few games.