Any truth to my feeling that MM carts are possibly better than MC on rock recordings?

MC cartridges have very diverse sounds across different brands (sometimes even within brands), so you can’t really make a blanket statement for that side, at least. Rock sounds best on my Koetsu (Coral, Blue Lace), for my tastes, I know I like it way way WAY better than Ortofon 2M Blue, Black (though I know not a fair comparison). I tend to gravitate to MC cartridges that are warmer and can rock well - Koetsu, Shelters, Benz, Ortofon Cadenza Bronze (NOT the higher end Ortofons!).

You said you have a Quintet Black, which uses their shibata on boron or sapphire. Funny, I didn’t like their Jubilee MC (shibata on boron, and predecessor to the Cadenza Black) for rock, or much of anything else force that matter - but especially on rock. Generally anytime I’ve heard an Ortofon with a more rigid boron/sapphire cantilever I’ve though it fell kind of flat with rock.

Also a tube MM stage plus SUT plus MC cartridge can have amazing warmth and punch; awesome for rock. Pair the right stuff together and you will be in rock Valhalla.

I think this is just a quirk of the Ortofon MC line. Some 12 years ago when I "upgraded" from Ortofon Kontrapunkt "c" (Cadenza Bronze) to Jubliee (Cadenza Black), I definitely did not feel as engaged in the music - and yes, rock music fell flat. Then I upgraded to Windfeld MC and it helped but did not mitigate all my issues - the lower Kontrapunkt "c" was a better all genre listen. That Jubilee uses a Shibata like your Quintet Black, btw. I definitely stayed away from the Cadenza Black (Shibata) because of my past experience here.

But that is NOT at all representative of MCs!! I now have many other MCs (Koetsu, Shelter, Benz) which all sound beautiful and satisfy completely with rock and heavy metal. I borrowed a VanDenHul Colibri XGW and that was great too. I even now have a higher-end Ortofon that I love too: the A90.

To be fair also, part of the problem back then was certainly in my phono stage. As you move up in the Ortofon MC line, the output level also tends to go down, and going from a heathy 0.4mV output to 0.26 can be trouble if your MC phono stage or SUT is not well suited to that. My old phono stage back then (Sonic Frontiers Phono 1) definitely lost punch and dynamics below 0.4mV.