Venice tanked in the only game I've seen them in, so far.
Around T+200 and they had two cities... Venice, and Zurich. And to the best of my knowledge, no one had conquered any CSs they had puppet-ed.
As chazzycat pointed out, I'm not really sure the AI can be tailored quite well enough to Venice's unique possibilities and limits.
As for human players, between Venice and Austria, I'd probably pick Austria. Human players can use Venice quite well, but after having finally played BNW a bit and seen just how easily disrupted trade-routes can be if someone is intent on disrupting them, I just am starting to think that Venice may suffer in multiplayer because they will struggle to actually keep the large number of trade-routes they get from getting plundered.
Overall, Venice works best as a human-played civ against AIs, which is the normal scenario for many folks, so I see Venice as being top-tier in single-player, but maybe just mediocre in multiplay.