Yeah, uchiha can take on multiple opponents, but not multiple opponents at the same calibre. Madara, Izuna, both uchiha's that have unlocked their MS abilities were able to fight in the wars because they were up against the average shinobi. If it came to multiple opponents who are close to their level, or people who would force them to use their MS techniques, then no they could not fight multiple opponents. Don't forget it wasn't like Madara and Izuna vs an entire clan, it was Madara, Izuna, and the entire uchiha clan vs another clan. For all we know, the uchiha clan could have outnumbered the other clans.
Madara and Izuna were never said to have huge chakra's, they had exceptional chakra, as in power, not quantity. And fighting in a war =/= fighting many people when you have your own army fighting with you.