Masquerade: The main difference between having a UA that gives you additional benefits to the Great Merchant and having a UGP (being the Merchant of Venice) is basically that it's another option for an allied CS to give you with the associated policy (now Finisher, as I understand it).
Right now, Venice is disincentivized from taking Patronage because it can't get MoV from it (well, any more than anyone else can) and it shouldn't want CS allies anyway, in the same manner as Austria.
Personally, I think that Venice is, if anything, overpowered, and the MoV is one of the lackluster elements of the Civ. (Buying in puppets is quite probably worth not being able to create your own settlements.)
Rather like Mongolia and its Keshik determining that it can't get other useful abilities, buying in Puppets is the overarching strength of Venice, and dictates that it can't get many other strong benefits.