Why would I ever want to use a browser based solution instead of local VMs? If you're worried about VM escapes then you have bigger problems (and hopefully a full time security team supporting you).

Edit: I realize in hindsight this comes across as overly negative. I think those are great solutions to have available for when you are working with a suboptimal local setup for whatever reason. I just don't think they're the default choice let alone any sort of ideal to strive for.