I'm not going to dwell on the details and the technical aspects. Others do that way more competently than I. I'll just give my quick opinion after trying RIOT.
I've really been struggling with finding a suitable analogy. The best I've come up with (and it's flawed as fark), is that RIOT feels a little like semi sleep-walking while on an unhealthily high dose of non-stimulants in a post-apocalyptic deserted environment.
There's no ambiance at all. It's almost completely silent, there are no people and no cars. Almost no sensory input except stuff which intuitively does not make sense, so you semi-ignore it. You wander aimlessly around, not really knowing what to do, where to go. Annoying (Y/N) messages pop up once in a while, but you don't really know what to answer and what the consequences are. Does the toxic fog feel toxic? Kinda, but I think I have this suit... sometimes. Whatever.
The very best analogy might actually be, that this game mode feels exactly like the opposite of what a riot really is. I know that RIOT is an acronym, but I honestly don't think I could make this game mode feel less like a riot if I tried. And that is neither a /pop issue or a "bugs aplenty" issue. It's just... bland.
If RIOT was to have a threat "colour" like normal mission districts, it should be beige.