Well we did it. We have our UBB installation going across 4 web servers. We tried replication and that did not work very well for admin functions. Worked ok though for avatar uploads and such as it would distribute to all servers in less than a minute. But when it came to changing admin settings if you were not on the master server (luck of the draw with a load balancer) we had a large chance that the changes would not save.
So we ended up doing an NFS share at the top level forum folder. It seems to work ok. It's not the best solution in my opinion but it's what we have at the moment.
Also. Since upgrading from 6.5 to 7.2.2 we've seen double the page views and a crap load of thank yous from our members. We are at over 1,000,000 posts now and 60,000 members with around 150 new members per day. Oh and about 8,000 page views per hour.
So UBB can work on a large scale. We're proof.