Would allowing all Unearthed Arcana create rules problems?

I’m thinking of running a campaign where the players don’t have books and I wanted to recommend that they use the SRD until then, but also use any Unearthed Arcana material from Wizards’ site if they wanted.

Would allowing all Unearthed Arcana material create any balance problems or fundamental rule contradictions?

The main reasons being would be to allow players to use free material without worrying about buying books just yet, but also having available more interesting stuff to work with.