beamdog introduced 2 changes that somehow affect the gameplay.
the introduction of sod that lets start with much more xp, in a phase like chap 1 and 2 where the xp gain from quests and kills is lower then later, and the new npcs.
that have some special characteristics, two of them are long personal quests, that can be done only with he npc in the party and the introduction of some new powerful items that can be obtained only playing the quests. imo are created too special, with long quests, in the original almost no very important item can not be obtained without having in the party the npc associated to the quest where you loot it, and almost all the relevant quests associated with some original npcs can be done without them, you don't need to recruit nalia mazzi, keldorn or haer dalis, they are associated with quests that are long, interesting and rewarding, but don't need them in the party for the quests to happen. for korgan you loose a single battle just exploring by yourself the area and quests like the cernd one or the jan one are not comparable with the new ee ones. they have also on average better stats then the original npcs, mr tob excluded.
the long and interesting ee npc quests xp is huge, stack with other new npc quest xp and with the one importing from sod.
starting with 400k xp more then the one bg2 is designed for and adding the xp of 2-3 new npcs quests and the items you get affects the game balance in a very relevant way, as is true that some enemies level up with the party, but not all do it, many bosses don't, and in my experience (from using often tactics that over level the party and for having runs with many quest mods that introduce the same xp problem) the enemies absolutely don't keep up with the party raised power.
the difference is that to use overleveling tactics and introduce quest mods is a choice of mine, i am aware of the result and intentionally seek it, to play the whole saga with the new game between bg and bg2 and playing some npcs that are intentionally made more appealing and rewarding then the originals does not necessarily mean that a player wants to intentionally alter the balance of the game.
this does not imply that the introduction of a bridge between bg and bg2 or new interesting npcs is necessarily a bad thing. i don't own sod (and don't want to be spoiled about it) and the only new npc i played until now is neera, that i liked very much, good plot, interesting battles and good banters and romance. i have still to try the monk and the furry one, probably i will never try the evil ones as i can not rp such level of evilness, but this is a problem of mine, not a bad implementation of the game.
by the way if a player want to use some quest mods and some ee npcs, for the new content, not to alter the balance, and is importing from sod, the xp problem is even more exacerbated.
the introduction of sod and new ee npcs it is not a bad thing, but introduces a balance problem that has not been addressed in any way.
i would like to hear your opinion about the altered balance problem and if you have solutions for it to propose. i suppose that installing in soa a mod that slows the xp gain can be one, the player can set how much is slowed depending on how much ee npcs want to have and on other factors like playing also quest mods.