Gasly was horrible at the RBR, yes, doesn't mean he's bad overall, still too early to judge him. Bottas is what you would call an average driver, doesn't have what it takes to be a champion. When you have the best car on the grid by far, the minimum you should do is finish at least second. He was never a threat to Hamilton after the first what, two or three races? The reason RBR is not second in constructors is because their car is not the second best, simple. An amazing driver can only do so much with a car that is not the best. Senna didn't win a championship with the Lotus, even though that was a pretty good car but not a championship winning car, kinda like RBR. We will only know for sure if Max is on the same level of Hamilton if he got at least a car of the same potential. Even then Hamilton would still have the advantage or experience that only comes with time, so we could only really compare pure speed.
Ferrari got a much better car after the summer break, cheating or not I don't know, but it was slightly better than RBR before and way better after. So yes, RBR is the third best combination of chassis/PU.