There's a tradeoff of tag players having more health to work with but also taking a more damage, while the reverse applies to solo. However, if the tag player isn't that skilled or is somehow overwhelmed, the extra bar can be totally negligible.
"...I do not fear the Klingon Empire!" -- http://www.youtube.com/watch?v=zh4JbLhH_8w PSN :Raven-Ghosthawk
Yeah, statistically. You have about 1.5x a normal health bar, which is still less than their combined health.
you also cannot do red bar damage outside of forcing a tag crash.
The main advantage to going solo is that you get rage a LOT more often.
I'd say characters like Paul, Bob, etc, that can absolutely wreck you off of one good read in rage mode, are probably not at a disadvantage at all. That being said, a team like Jin/Nina, where both characters get rage faster than normal because they trust each other and have a few of those scary high-damage pokes themselves, offsets that advantage; there are lots of variables.
"Siegfried: He has no need for words, he speaks with whiffs." - Wandrian Official General of the Fire Emblem: Awakening Boards