Balance isn't really a big concern until it hits game wrecking or overshadowing the other players levels. In 5E that's usually level 17 or 18 comparing PCs. Assuming someone knows what they're doing and tries hard enough.
I think it can be a bit more of a concern but still be in a fairly loose good enough category.
In playing Rifts you can see there are wildly variable tiers of character classes in the core book, powerful MDC classes like the Dragon, the Juicer, the big power armor one with the giant boom gun thing, the Ley Line Walker and Techno Wizard and the Super Psy. There are some mid tier classes with OK but inferior stuff compared to the top ones, then bottom tier things like the scholar and city rat who are without MDC stuff and are basically normal people running around next to super powered stuff.
In my group we found it plenty of fun to play Rifts but we were all always top tier classes, there are different options with different mechanics and narratives and feels who can hang together in the same fight and it is close enough and was fun. It would have been terrible to play a scholar and be blown apart automatically in the first hit of a combat.
I consider 4e the best balanced D&D of the various editions. 5e is not as balanced, but is close enough to be not a big concern. 3e had balance as a goal but the execution was not always great and the options and set up allowed enough imbalance to become an issue fairly quickly. Pre 3e D&D has a number of balance issues starting with the thief introduced in OD&D supplement I Greyhawk and running through the whole set, particularly with AD&D stat reverse bell curve bonuses, gatekeeping more powerful classes with high stat requirements, and balancing low level stuff against high level stuff.
In pre-3e AD&D it is easy to avoid playing the low tier not-balanced thief and have plenty of fun options. I think it is better in 4e where IMO a thief is a fantastic class balanced well against other 4e classes and is a great choice for a typical D&D adventure.
Where balance becomes an issue will vary individual to individual, and the line of good enough balance will vary individual to individual as well.