I am not that skilled at using Roll20; I don't have a lot of experience. But I can speak generally comparing R20 and Fantasy Grounds "Unity," the newest version.
FGU has no integrated voice or video (R20 does), so if you want to be able to speak and/or see one another, you need to use another service as well as FGU. I use Zoom, others use Meet, Discord, and so on.
Both have dice simulators.
FGU has -extensive- rules integration with many systems (though the only Traveller versions supported are Mongoose 1st & 2nd edition). By "support" I mean all the math for all rolls can be handled by the platform, which is sweet. It doesn't "enforce" anything - meaning you have a lot of freedom and making mistakes is okay. FGU also has "line of sight" capability, which means players on the map only see parts of the map they have line of sight to, on maps that someone has enabled. Kinda cool, the players not being able to "see" around corners or into rooms they haven't entered yet.
FGU has a pretty steep learning curve. It took me separate tutoring sessions with each of my players to help them, then several sessions for them to become comfortable. But now that they are... it's pretty sweet.
Roll20 (to my knowledge) is more like a shared map, easier to learn and manage if you don't need it to be more than that.
FGU feels more like something you pay for, whereas R20 feels like something a few people put together.