Yes, it is a mess if there is a failure, but that probably won't happen.
I get my flush water from the same line that the sink drain is attached to. For some reason, maybe because the T in the line is below the waterline, I never suck any air from the sink drain when flushing. This came in handy once when all of the water blew from the west end of Lake Erie where we keep the boat (the water level dropped 50 inches, yes you read correct 50
inches) during one storm and the hull ended up resting in the muck on the bottom of the marina. Needless to say, the only way to flush the head was to fill the sink before pumping.
As long as there is a pump out station area nearby, you won't need your own pump. The pump-out station is a giant poop sucking vacuum that sucks the tank empty. Check out this link: Installing a Head by Don Casey
There is another one by Peggy Hall, the HEADmistress. You can find her on Sailboatowners.com. I think she also has a book.