Any caster who stops at level 6 will miss out on 4th level spells (and obviously 5th level spells too).
Also, certain feats require you to be at a certain level? Where's that in the rules, I don't see it.
It's under each of the class descriptions under ability score improvements. Oh I'm well aware of what I would give up with some builds, by stopping short. Wizards have always been my favorite class, and I've been playing them for over 40 years. For instance I'm working on a Red Dragon Sorcerer 8/ Evocation Wizard 2. I would lose 5th level Arcane spells (but not the slots). I would also just come up short of adding 2 2nd level spells to my Wizard's spell book. But I would gain Sculpt Spell, which really fits. And I get both of my ability score/feats. With the Warlock 5/ Wizard 5, I just decided the Armor of Agathys + Arcane Ward, Devil Sight + Darkness, and Improved Pact Blade was worth sacrificing 4th and 5th level spells. Though I'm still not set on it. Banishment works really well with this build. Dang we need a level cap of 12.
