Research reports overwhelmingly that the vast majority of synesthetes see it as either neutral or advantageous. There are rare cases of sensory overload for very intense synesthesia, and a possible but unconfirmed connection to lessened spatial and/or mathematical awareness, but those are so far the only real downsides listed aside from lack of understanding by non-synesthetes.

And there are preferences influenced by synesthesia--I have a passionate dislike for the letter "c," for example--that make us a bit eccentric.

But describing it as a "disability"--something that impedes everyday functioning--is a stretch no matter what.