I've run several games for friends who live in other cities on Roll20 and the current campaign I run is there. There are more pros and cons than this but as you only asked for two, here they are:
- all the important features you need to play are free and if your DM does decide to pay, everyone in the game can see the paid features like tokens and modules.
- It's relatively easy for everyone to jump onboard and play, and with a little prep, it can speed up things like player handouts and rolling as there are in app dice rolling mechanics (if you aren't superstitious like my players about them)
- In my experience, there is no method to show verticality. I.e you are going to have to remember how high flying enemies like dragons are.
- Getting the in game grid to line up with maps you upload can be a little finicky at times. There is a tool which is supposed to help set the roll20 grid, but I tend to find that he grids still slowly drift due to the difficulty in getting the grid size exactly right.
My best advice is for the DM to get used to the system and try to set up as many features beforehand as possible. Spells can be set from the compendium which makes rolling easier and you can preset tokens to character sheets and set them with things like HP bars as well. I find running games on it works just as well as my in person ones, though sometimes things can get slowed by technical difficulties.