I think this is a different topic, but encounter building and balance has been more art than science in all editions.
I allow all feats, multi classing, I assume PCs will have a legendary item by 20th level and so on. I do limit access to a small number of magic items until higher levels (i.e. boots of flying).
All I can do is say that I have no problem with gonzo PCs, I always have infinite dragons. Even if some of them have several levels of sorcerer with still spell and powerful illustrations. Or just take a page from Tucker's kobolds and play the enemy smart.
TLDR: balance issues are nothing new in my experience.