Nitrobetting and mBet have absolutely no activity in their ANNs, and I doubt many people on Bitcointalk use them. Hence, it's only logical that you won't find scam accusations against a site with very few active Bitcointalk players. If you take a look at their ANNs, the only posts are those from their forum representatives. Betcoin is almost the same. The last 3 pages are almost exclusive to match previews and promotions posted by their forum account.

Stake, on the other hand, has a lot of activity. I haven't looked at the content of those scam accusations you mentioned, but take any scam accusation you see with a grain of salt. I visit the scam accusation board occasionally. There are valid accusations but not as many as you would think. There are many fake ones posted by abusers of all kinds who are angry they got caught and would like to see the world burn. Some accusations are totally baseless or don't share a single piece of evidence.