I think the real problem is the writing; characters are little more than their specific “type”. For instance, both Carol and Heather are basically just “sexually neglected mom”; they have slight differences in personality, but neither is particularly well fleshed out. However, I firmly believe that you could have a game with twelve love interests that are all “sexually neglected moms” that wouldn’t get boring in the slightest, so long as the writing was exceptionally solid.
I believe the cause of NLT’s problem is twofold: first, the “porn comedy” nature of the game lends itself all too easily to commonplace “porn writing”; second, the formulaic nature of the series makes it easy to fall into character stereotypes—they’re ticking off boxes from a checklist (much like Japanese harems that have cookie-cutter tsundere, big sister type, shy glasses girl, catgirl, etc.) instead of creating real people. Although, I suppose this could be a chicken-or-the-egg scenario: is it the formulaic game that leads to poor characters and weak writing, or is it the weak writing and poor characters which lead to formulaic games?