Well - there are limits on Venice as well. It all comes down to play style. Firstly, beyond the first free Merchant of Venice you will have to work at getting further ones and there's only so much you can do to speed things along. Also bear in mind that you might be skewing your GP generation towards merchants rather than GS', GEs, GWAMs etc.
On top of this, even when you do get your Merchant of Venice it might not always be easy to get it to the City State that you might like to claim (particularly if you're at war or there are Barbarians still knocking around) - and even if it is, it still may take a while.
True, Austria have to be Allied and Venice doesn't but all this means is that as Austria you need to work at getting alliances with the City States that you want to buy (which isn't too difficult most of the time) and you also have to work at building your treasury to pay for the marriage. You can also buy that City State wherever it is in the world and even if you are at war.
So I can see your point but I don't believe that Venice have it much easier than Austria - they just go about things differently. I think it's easier to plan your CS strategy with Austria than it is with Venice (at least with regards to timing and placement).
Anyway, this is just my opinion of course - we all have different styles of play.