I think I'm pretty solidly in the camp of "Discord is for banter and quick discussion", while the OOC is where the GM or players make more official statements.

I personally don't mind being on Discord, but I have noticed that it has a tendency to leading to a sort of cliquey environment where it's we "the Discord users" in an RP as the driving force because we're regularly chatting with the GM and orbiting on the outside are the people who only post in the IC/OOC (which feels far from ideal, not everyone can or wants to be on Discord all the time or even some of the time).

There is also the downside that if you (the players/GM) spend much of your RP activity on Discord then it can also seem like the IC/OOC are pretty dead (which isn't maybe an issue if you have your core group of players, but if you're hoping to get more/new people it can seem strange when the IC/OOC are virtually dead for months at a time).

In an ideal world, I think it's good to try and balance your use of both OOC and Discord, leveraging the advantages that each can bring, and in turn reducing the disadvantages that relying solely on one or the other give rise to.