I dunno, that kind of depends on when you're comparing them. Kaka for starters, I've heard was good at Milan, but I only ever saw him during his Madrid days so based on how he did there I consider him massively overrated and I'd definitely put Neymar above him.
Ronaldinho is similar. I did see him at his best and I'd put him on the same level as Messi and Cristiano Ronaldo during that time. He was only at that level for a handful of season though, and non-peak Ronaldinho is definitely comparable to Neymar.
Since everybody considers that Neymar is in his prime years as a footballer, all comparisons take in consideration the best years of the players I mentioned. Neymar is way under all them, really, they are not remotely close. It's really a shame that you haven't seen Kaka between 2003-2007 at AC Milan; Ronaldinho at the same time playing for Barcelona and specially Rivaldo and Ronaldo in the late 1990s, by what you say, I can deduce you are too young. In a way, your comment agrees with me, Neymar in his prime is similar to players near retirement.
Not so much that I'm too young, I saw some of prime Rivaldo/Ronaldo, more that I've never really watched Italian football. In the UK the main ones that are televised are the British leagues (obviously), Spanish leagues and UEFA competitions, so I've seen little if any of the Serie A and Bundesliga.
Still don't think it's fair to just compare prime vs prime. Longevity is definitely a factor in how good a player is and having a better overall career may well be better than just a handful of fantastic seasons.
That said, just looking at the stats there should probably be some question marks around Neymar's longevity too. He seems to be playing a lot less games per season at PSG than he did at Barcelona and it's not as if he's that much better either. Still, I think the comparisons to Kaka and Ronaldinho at least are fair, even if he's not as good as they were in their prime, his career as a whole could certainly be comparable to them once he does hang up his boots.