The CR system is a mess, and not terribly accurate. Often it's entirely arbitrary. We do our best but CR certainly doesn't equate difficulty perfectly. The net result is a huge variance in difficulty in different zones. Next is the spawn system which was designed for a single player RPG not a PW which loves spawning loads of effortless stuff... The net result is even for characters with a combat build you can be down to 1 to 2 viable hunting areas at any time. And characters without a combat build....
Next there is a huge difference in what is and isn't a combat build. An optimized wizard or sorc is different from a cleric/7 level WM, which is different from a ranger/rogue. Undead make things easier for paladins and clerics and harder for rogues. Elementals suck for everyone.
And the final icing on the cake... how exp works in NWN. EXP scales very gradually compared to mainstream MMOs because DnD was not designed as a mainstream MMO. Getting 40xp per kill is just as good at level 1 as it is at level 24, so that makes it very hard to create a lower exp lower difficulty hunting option.
The bounty system was an attempt to help solve this by giving solo players a way to get exp and cash, but there are a lot of things about the bounty system I'm not satisfied with. I've considered changing it to two 'tiers' of bounties, one of which would always spawn outside dungeons and provide lesser rewards, the other which would always spawn in dungeons and provide greater rewards. There is another system I've mulled over in my head as well but again... the question is finding the time and effort to put them all in.