I can only talk about my experience but I noticed a few things during the VTT sessions I did as a GM and as a player compared to real life ones (and I did a few):
-VTTs are tiring, particularly for the GM. We play without camera so we don't see the others and I think that's a big part of it. Having to keep constant attention to what's happening without that connexion is hard. Particularly it's very hard to not start talking at the same time as other players without visual cues forcing extra mental work to make sure everyone can talk.
-VTT are easily boring. If a conversation between the GM and a player or 2 players goes on too long it's very easy to get bored. Empathy is harder without seeing faces so everything seem more distant. Also only one conversation is possible at the time. Usually you can quickly whisper something to someone without losing track of the main conversation, here you can't.
-VTT work well on the other hand when doing tabletop strategy. During fights there are way less problems than the RP phases. Moving tokens is intuitive and having automatic sheets that do all the calculations for you is great.
-Some people can't deal with VTT: I had a GM that didn't want to continue GMing his table with VTT after 2 tests that werent great. Basically the first problem I discussed was too much for him. He felt physically exhausted after (and during) a table. I also had a player that didn't want to play (other table with an other GM) because of a combination of the two problems (tiring and boring).
To be fair the two campaign that were stopped were heavily RP based with very few fights.
I am currently GMing a lighter setting where fights have a huge role and I spend a lot of time crafting maps and tokens so my players have visual cues. It seem to work very well.
Long story short I feel that VTT work well for a campaign centered around strategy and fights, and a lot less for one based on RP and drama.