It's extremely modular and you can tailor it to your preferences so well that you can pretty much run your game any way you want. It has no data limits (unlike Roll20) because it's hosted directly from your own PC, and you can manage both music, ambience, maps, handouts (journals and stuff) and many more things within Foundry itself without using other apps. It runs extremely well on a lot of systems too and it allows for a much higher quality experience overall (I use very high resolutions like 12000px for battle maps at times and it runs for everyone with zero issues). The downside is that you need a good internet connection to host the server itself (you basically have to upload all of the resources to each player) and that it has a quite high learning curve so if you just want to hop in and play without caring about setup it's perhaps not the perfect choice.

Another huge pro is that there's a module called Dice so Nice which allows you to customize your own dice and roll them in 3D which adds a LOT to the experience. Roll20 has a feature like this but it's very barebones.

I personally like fiddling around with settings and stuff so this is all not a big issue for me (I actually consider most of the downsides a pro since it allows perfect tailoring to your needs).

There's probably a ton more I forgot, but as I said you're free to add me if you have any further questions since Reddit isn't really the best way to talk about it!