Some mpcs just work better on some tubas than others. But it does also depend on the player and what works for him/her.
As an example, I had been using a Kelly 18 in one of my huge-bore rotor BBb tubas.....until recently. That mouthpiece works well in all my tubas, but just OK in that horn. The intonation was good in the middle and lower registers, but flat up and above the staff, even with lipping. Response was OK, not great with that mpc. Anyway to make a long story short, I find that the response and intonation on that tuba are improved with wide, bowl "coffee-cup" mpcs like my Benge 7 (Bach 7 UMI stencil) and drilled-out-backbore Schilke 69C4. The response with the coffee-cup mpcs is a little smoother/more forgiving and the "flatness" of the higher register is noticeably reduced. Some of those higher notes tended 20-25 cents flat; lippable to maybe 5-10 cents flat before getting into "chipping-note-to-next-partial-territory" with the K18 and other medium-sized mpcs. But with the wider, coffee-cup mpcs, those higher notes are now maybe just 10 cents flat and lippable in tune (with room to spare a few cents "sharp" if need be). Slurring & legato playing are a bit easier too.
In other words, the intonation & response
tendencies are still there on the horn, but definitely improved with the right mpc. I should also note that the Benge 7 goes in a bit further into the mpc receiver than some of my other mpcs. So there is definitely some merit to the mpc/receiver "gap" that is talked about on here. I think that if a mpc sticks out too far, it might throw the intonation/response 'alignment' off a bit.
I guess that's what ppl mean by "mpc sensitive"
There was a great mpc diagram in the old WW&BW catalogs from 15+ years ago. It talked about each part of the mpc and went into detail about what each part does. Wish I could find it online
