Yes when I done some bjj/mma training back in 2013-14
I was normally good at the start of sessions, as I was about 220 lb at the time and quite lean with a strength background.
Most fighters there would usually pull guard on me and work off their backs. I was a beginner and not good at submissions, other than the guillotine, which I surprised a few of the guys with. I think the fighters liked me to smother them, as it was good practice to have a bigger, stronger guy put them under pressure, before they get ready for a fight.
I was tapped when gassed, but not normally when fresh.
Mind you, the gym never had anyone higher than purple belt at bjj and most were just beginner or blue belts.
Even the guys fighting were semi-pro, bar two guys that were pros.
I do know for a fact that size, Strength and speed helps loads, but technique and stamina are much more important for rolling.
Only because of my bad eye injury (orbital fracture, that has left me with permanent double vision) from kickboxing, I would be all over mma.
In fact the new gym I am at now, is run by one of the former fighters and he is going to start some classes again.
I doubt because of my age or injury, that I will fight, but I would really love to give it a try at some stage. I just don't know if the risk of losing sight in one eye is worth fighting for no financial reward.
To cut a long story short, grappling is seriously hard work and it took me some time to get the endurance for it.