For those who think boxers like Tyson would mop up in mma you should consider the experience of Stepahen Oullet - a former top 10 ranked boxer. He competed in the UCC, a Canadian promotion that’s a step down from the level of the UFC. His first fight he was fed a fish - his second he met Pain Peters. Pain is extremely tough and has some skills although he would be the first to say he’s not an A class fighter (or even B class). Pain was able to take Oullette down numerous times only to be stood back up due to Oullette cheating (ie headbutts and biting). While Oullete was eventually disqualified Pain dominated Oullete and only got hit hard once (although it was one solid shot).
As for those who think Rickson is the greatest bjj fighter of all time - I think you’re right. Greatest mma’s fighter - no way. Rickson may well have been but he never had the chance to prove himself against decent competition as no one with skills arrived on the scene until after he was past his prime. Rickson is one of the heroes of the sport though.