I beat BG1EE last week for the first time with SCS installed, but only the improved AI components (smarter mages/priests, basilisks, sirines, etc), no improved encounters. I consider myself a novice and the difficulty for me was about right. Like in vanilla, the early game is brutal, but it gets steadily challenging once you get a few levels. Naturally, the difficult drops once once your acquire late gear (that's what improved encounters is for, I presume), but I loved that SCS will still punish you if you get lazy or careless even if you are strong. At no point I felt cheated.
It's been years since I last played BG2 and SCSII is a very different beast, in a good way. The jump in difficulty is blatant! In SCSI, enemy mages would cast all sorts of crowd control and leave you helpless for their petty fighters, now they will take no hostages!! Alchemist fighters are also nasty and ninja thieves are a new force to be reckoned with! I certainly need to update my knowledge about spell combat.
I have done about 7 quests so far and before I go on, I'd like to know how hard are the 'smarter enemies' component? Especially dragons and fiends. I have these installed and I'd be very interested if they behaved more realistically (which is the stark premise of the mod, at least for me), but if that made them much harder, I'd be definitely biting more than I can chew. I barely noticed the changes from smarter enemies in the first one, and some of them proved to be harder in vanilla (sirines, for example).
As a side question, do you guys install spell fixes and tweaks, like better Improved Mantle, nerfed Iron Skins, etc? What about overhaul from PnP mods, like aTweaks?