My biggest concern would be improperly welded pipe.
I've seen that before in houses where the homeowner was doing interior work, and even out of some regular plumbers. They figure that vent isn't going to actually have water draining in it, so they do a shoddy job putting the vent line together. One house, the vent lines were just stuck together, not glued at all.
As for the tub draining, you should not have an issue, as tub drains allow air in from the overflow.