Personally...
Hot-shoe guns are have enough power for most things, but lack of modelling lights makes setting things up just right much more difficult. For example, trying to angle a reflector when you can't see what's happening as you do it, is very tedius - just a few degrees one way or another can make huge difference. I think this makes learning harder, and you're less likely do try different things.
The recycle time is much longer, like three or four seconds compared to say one second. I've missed no end of good portrait expressions that way, which is frustrating and makes you look amateurish.
You can get around this by turning the power down, but then you start chasing your tail with low output. The good high ISO performance you get with a lot of cameras goes some way to combating this, but sooner or later you'll run out of puff. Best hot-shoe guns have the equivalent of about 60-70ws output.
Hot-shoes guns are not cheap, especially after you've added some decent light modifiers, triggers, flash meter etc.
Hot-shoe guns are so small and convenient, and portable/mobile. You can put them anywhere, and TTL-auto systems make some very tricky exposure situations a simple matter of switch on and shoot.
I use both studio lights and hot-shoe guns, often together