Dunno man, every ARPG game is never supposed to be balanced early game. End-game is where its at. The first 20 or so levels are going to be different for variety of classes. I managed to get a Barbarian on Hardcore to level 15 before the servers shut down and a rogue to 25 also on Hardcore.
It's impossible to balance a loot based and skill-tree game for the first 20 or so levels, it just never works. Diablo 3 is same way. However, when you reached 70/max it all made sense, because everyone had access to their entire kit, and at that point it was a matter of getting the right items and creating a farming build first until you've farmed enough to get a end-game build. Why are people so surprised about the balance here? And yes, I played on World Tier 2 which was max difficulty. I've lost a few barbs and rogues, but I managed to learn the classes, their skills and itemization within the 30 hour or so period of my playtime and had a blast doing it.
Yeah, I'll agree melee characters are always more gear dependent and they are usually forced to take damage when doing damage because they have to be up-close. But I wouldn't say Barbarian is severely underpowered. We won't know until everyone levels toons to the max level which is a 100 in Diablo 4. I can see Barbarians being the weakest once at the start sure, but that doesn't mean they are unbalanced once they're max level, but we can't say that either because we haven't seen end game. Once the game releases, everything will be a lot more accurate.
Out of all the characters that I've played I found Sorceress to be doing the most damage and the easiest to kill with, but that's just naturally cause they are the glass cannon nuker of the game. Rogue, while a lot of fun and good, has severe energy regen issues. The build that I run on my Hardcore Rogue didn't really perform super well until level 20 because that was when I finally got one of the skills that I really needed to complete the build. However, it was a good enough build to get me to 25 and kept me alive even though it was a bit slow, it did its job.