Like with any tool, there are going to be downsides. I used to not have any sort of chat server for my RPs, largely since a lot of my RPers were already in Lady Amalthea's chatroom and I didn't see a point in making a second general chat area for largely the same people. However, once I saw how handy discord was for a GM, I created a RP server and I have it organized by RP. It also includes a reminders and announcements section, as well as behind the scenes areas for my Co-Gms and I to discuss. 

Benefits wise, I have seen players become more invested in the RPs. We aren't just a disconnected group of strangers who occasionally interact in the OOC. People know the person behind the username and I think that takes away a lot of the ~nervousness~ about posting in a RP and makes people more enticed to continue on when there is a lull in a storyline. It has allowed for players to ask me questions quickly, as well as for me to send out announcements and remind people about posting in a setting where I know everyone will check at least once a day. A lot of great ideas and memories have come out of discord.

Negatives wise, cliques are definitely a thing. It's up to the GM/server owner to enforce equal participation and opportunity. People will naturally prefer old friends over new ones, so it's up to the server owner/GM/chatroom staff to make sure that no one is being excluded or left out. You can't just hope for an inclusive environment. You need to make sure one happens. I have also found that fights break out easily on discord between players, so to counteract this I've appointed a chat moderator from one of my trusted RPers and that has been helping keep things civil whenever tensions arise (we're human, tensions will happen). 

I myself really enjoy the discord server for my RPs, as well as servers I am in for other GMs. If it works for you, great. If not, then it's no big deal. Everyone just needs to follow what works for them as a GM - and this goes for RPers too. If you don't like discord servers and the GM requires you to be in one, then that may not be the RP home for you. And there's nothing wrong with that in my opinion.