For me, balance issues are never the party vs. the monsters. I can tweak the monsters to make the encounters more challenging. Players want to feel powerful! Give them the tools over time to build the character they want to build.

It's power disparity within the group that has always been the biggest problem. If you have players that are very knowledgeable about the game and know how to build optimally, other players may feel like their character isn't good at anything because the more purposefully built characters will seem to be able to do so much more.

I usually have to balance that with custom feats or items to even things out. Disallowing multiclassing in 5e in my new campaign also helped. Too much front loading that encourage dips for the sake of dipping.