The thing with Venice is that it's very powerful...but requires a lot of risk to make use of that power. It can completely ruin your day as an opponent, but is also always in a very precarious spot. Obviously, the way you want to play with them is by gearing your focus to as many Great Merchant points as possible, dipping heavily into gold boosting bonuses. Played right, they are capable of amassing absurd amounts of gold and getting some fairly nice territory in a fell swoop, especially if that CS starts near a Natural Wonder.
But here's the thing. That comes with a hell of a lot of limitations. The most obvious being that your initial settler is all the city choice you get. All the land that you explore from then on, all the optimal city sites you're trained to see, you just have to look at those and sigh as the AI maybe settles those but probably in dumb places. For a good portion of the game, you're very defenseless. You often don't appreciate the value of having a barrier of cities protecting your capital until you have Genghis Khan and Napoleon founding cities right at your doorstep. It's not like city-states are founded in circles around you. On most maps, they will be at the far corners of the land, isolated and detached, often on spare islands away from the mainland.
And all that focus towards MoVs means there's a good chance you'll detract from other Great People. You have to shy away from the immense bonuses Great Scientists and Engineers give you because they will increase the costs for your most important unit. And even then, you have to utilize your MoVs to gain territory peacefully, period. Buy out a city and you get no money from that action. This tempts one to instead do a Trade Mission possibly, taking away opportunities to control land. And forget about Customs Houses, you can't afford to utilize those. Pretty much you have far less opportunities to plant GTIs. Where other nations have their capitals filled with academies and manufactories, yours is left humble by comparison. Great People are no trifle to get. Sure, you can acquire about three merchants by dipping into Liberty, but that gets you four cities at a time where most players could get five or six, and choose where they are.
There's also the matter of Venice's biggest monetary advantage being entirely eradicated by embargoes and war. If you block Venice's trade routes, that's basically it. Their economy is crippled and most of what they have going for them is gone. They're reduced to a poor country with one moderately powerful but very precarious city, and scattered city-state puppets that they can't directly control except for buying in. Oh, and there's another thing. That buying in puppets may well eat most of their cash. Let's face it, puppets always produce things stupidly. They will spend 138 turns building a Stock Exchange when they don't even have a Workshop. And those city-states will comprise most of your territory. You will NEED to buy in them if you're to adequately defend them. 250 GPT is great for a normal civ, but less amazing for Venice when you need to constantly buy things in the great percentage of their cities. Still pretty powerful and versatile, but you have a lot more you need to buy.
Honestly, a thing I'd like to have them do is put the MoV's abilities in the UA and give them the Arsenale, a UB that replaces the Seaport and gives more production bonuses for boats and other things in general, a bit of defensive bonus, and maybe even does something cool and spawns two of any naval unit produced, though that could definitely be way too overpowered. But I like UBs and think the Arsenale would be cool and is very iconic of their culture and prowess with naval abilities, while the MoV's abilities feel like they could be moved to the UA since...let's be honest, the aesthetic concept of a Merchant of Venice feels a bit of a lame cop-out. The Great Galleass is also pushing it a little, to be honest. I like unique things that are immersive of a culture, and those two are...not.