Forgive my ignorance, I've got very little knowledge of combat sports, but wasn't UFC 1 a real world application of what would happen if you put BJJ up against boxing, kickboxing, sumo... where Royce Gracie ruined guys 25+ pounds heavier?
Heavier guys
who didn’t know any jiu jitsu.
A good wrestler with good striking skills and some basic understanding of jiu jitsu will be very difficult for a pure jiu jitsu guy.
Nowadays in the UFC everyone knows submissions, take down defense and striking so the pure BJJ guys have trouble. Plus, rounds with re-starts are another factor.
Ryan Hall is one guy who (I believe) can use basically just BJJ, along with his opponents‘ fear of Ryan’s bjj (nobody wants to take him down so he can throw crazy kicks) to win fights in the UFC.
Also one must consider that these days the vast majority of BJJ guys are doing “sport“ BJJ. BJJ for fighting is different (and simpler) in many ways.
If you go to a gym that teaches any self defense (or whatever you want to call it) based BJJ you’ll learn all you need ”know” within a year or so (you still need to train consistently to be effective). Basically all the jiu jitsu that is useful in a fight would be known by most solid blue belts (this is a broad generality).
The jiu jitsu that I do at the level I’m at now is training to do sport jiu jitsu against people who are good at sport jiu jitsu. Most of it is very technical and subtle adjustments make huge differences. A lot of it just takes time (applying pressure like ‘this’ or like ‘that” in this or that situation....). A lot of what we do would be really stupid to try in an actual fight. This is why I also wrestle, kick box and spar jiu jitsu with strikes (ground an pound) as often as my old body tolerates.
But, yea. If you know nothing about BJJ and try to fight someone who is good at it (or even an old mediocre hobbyist like myself) it’s probably not going to workout well for you.
Edit to add...
the Gracies set that whole thing up to ensure a Royce victory.
And they used Royce (instead of Rickson) because Royce was scrawny looking. Even his gi was worn too big and his weight lied about to make it seem more impressive.