My major gripe with VTTs isn't about any one in general. It's simply the nature of the beast.
And that is that once you're invested, you're invested. And I don't mean just buying books for each platform or licenses. There's also having to relearn a new system. For players, it probably isn't as great. But for a GM, learning DL is a hobby in and of itself. It's the sort of thing that you can really only learn by doing. And the more you do, the better you get.
I'm on Roll20. It's pretty good and I'm used to it. But I'm always on the look out for something better.
But how do you define "better" - an inherently subjective term?
What is keeping me is that I have already bought in and I am already fairly competent with the system. It's system agnostic so I don't need to worry about what system my players are on. That's a pretty big "pro".
I see two problems with LOS/DL. First, which is kind of unique to Roll20, is that as the 800 lb. gorilla, Roll20 seems uninclined to innovate too much. It's kind of content to be where it is. Second is the problem with VTT in general and that is what is a VTT trying to be? Is it trying to emulate a tabletop experience? Ostensibly it is. But when you get lots of bells and whistles added in, be it LOS/DL or ambient sounds or APIs for players to be able to do things like open doors, then VTTs veer towards trying to emulate a video game.
And that, I think, is a mistake.