Casters often feel at a massive disadvantage for casual fights. For a boss fight, casters are often the strongest, since you'll blow all your spell slots. But for smaller fights, you want to preserve your spell slots and cantrips simply cannot keep up with martials. I mean, a single attack roll for a spell cantrip vs getting 2-3 attack roles that also do more damage total? Heck, my strongest martials can usually do at least double the damage of a spell caster's cantrips.
Though at the same time, when I can blow the spell slot, no martial can outdo the AoE damage of reliable ol' fireball or the likes. Just I can't justify using my spell slots on a small number of weak enemies.
I’ve started to use my spells more and then resting more because you actually need to sleep to progress some story lines. I don’t think I did that enough for Act 1 and most of Act 2.
Also my gf and melee tanks many times also needed a full rest and I still had most of my spells slots so that wasn’t fun.
I agree with this mostly but I've got to add, I'll give up a spell slot for magic missile on a few low hp enemies any day of the week. Being able to get 3 guaranteed kills if your enemies are low enough hp and spread out is actually really nice.