Considering he won the HW title in only his 4th pro MMA fight and went on to defend it twice, I'd say he was pretty talented at least by HW standards.
Brock was amazing talent in his main strength area of top control wrestling and GnP. I give him full credit for that. He also seemed capable of learning BJJ (defense) quite well.
But there is a thing in fighting called 'getting exposed' and it is not myth or hype.
Houston Alexander, if given the right 4 fight path to championship, with the right Champ in place at the time, could arguably have been champ before his weakness was exposed. Once Houston was exposed he instantly became a D level fighter from the A some where surmising him to be.
Many guys who quit before your weakness is exposed do not quit after, which is exemplified by Conor/Poirier. No matter how much Conor hammers him (any top fighter) early, Poirier know he just has to survive, eat the beating and Connor will gas and he will win.
That was Brock. ONce it was realized that all any top fighter had to do was land one good punch on Brock's face and his entire fight falls apart he was never going to win consistently again, other than fights he could dominate and avoid that hit from beginning to end.
Had Brock not had the shortcut path to the Title, and instead was forced, like any other knew to MMA HW, to go thru a gauntlet of different fighters all with different styles, Brock would have been exposed far earlier, imo. I think he does not even make it in to the Top 10 as he just would not consistently win.