I've never ran an exoclick campaign, my current cash-flow allows a spend budget of $300 per day. Would I be best off choosing one or two sites and making them work, or running RON and looking for the best performing placements?
RON generally will require way more money so I would start buying 1-2 premium budgets and scale from there.
Basically, on any traffic source, assume RoN = 90+% of the traffic is going to have to be filtered out after paying to see that it doesn't work for your campaign. The bigger the traffic source, the more $ you are going to have to lay down to optimise.
This won't always be the case but definitely tread cautiously!
On exoclick you can only see a handful of websites bringing traffic thr othet website tjat did bring traffic yoi cant see they are placed under other
... why are thet doing that

I think RON is underrated on Exo. I've used it to find some great placements that I wouldn't have tried otherwise.
As for the big placements, test them methodically. Some of the major sites (like Xhamster) should be considered gigantic traffic sources in their own right. It's rare to make $$$ without getting to grips with each major site's individual quirks.