actually people mentioned in the forums that nicehash got problems with multipools, but if we can't use it with multipools, then what good is it? Blackcoinpool server is actually US based. So i don't see why nicehash would have problem connecting to it.
I've asked this to some pool admins in their IRCs and they advise to put a limit on the rented speed. This depends on the pool but usually around 0.3gh/s per worker.
Those that go unavailable(red) while waiting in the queue don't like to keep an open connection without any work requests and some pools don't want to change this behavior. Understandably, to save bandwidth(less likely) or minimize attack risk I guess.