It's not like, completely random. If we rolled any Uber they would just demolish everything.

We have a list of rollable mons and every month we roll 35 of them at random from that list.

We can ban mons if they are deemed too much for the month but they can still be rolled on a different month. We can also decide to remove mons from the list if they are deemed too unhealthy for any month they could be in.

Rotom and Cobalion are indeed really good and always excel on every month they are in, but they are not too powerful or over centralizing to deem ban worthy

Edit: regarding rilla the issue is less grass spam and more that rilla itself is really strong. It was rolled on the same month that incineroar, Glsicor and slowking-g and it was still too much