The issue is when it's cross server.
a cross server LFD system causes people to act like a-holes because there are zero consequences for doing so. Need on every item, go AFK, cause wipes on purpose, etc. The list is very long for ways to grief other players and with a cross server LFD, it is a 100% guarantee that it will happen.
If it is only server-wide, the dirtbags will get blacklisted very quickly.