I think people are so divided about the Black Prince because the "against" party is right when they say that it was never used in WW2 and too late in development. On the other hand, using and adjusted base of the Churchill and the 17pd gun as well as it's designated role in combat as an infantry tank, it visually and thematically fits into WW2.
That's why I assume that many people don't have issues with it being in the game: It "looks" like a Churchill, therefore it is believable to have been used in WW2. A Centurion for example wouldn't have that, this would be a completely "new" tank.
Overall, I am still against using it. While it probably would not break immersion (for 99% of players, it would just be a beefy Churchill), as others said, it would give a bad precedent. CoH2 using a ton of unicorn tanks is one thing, but at least you can imagine that you actually have one of those few in your specific battle even if chances are slim. The Black Prince however has never seen any front line. And while I personally could tick that off as "just another Churchill variant", I am afraid Relic would start implementing stuff that does not belong into CoH3 even by my comparatively lax standards, just because it can be sold well. Their CoH2 monetization strategy was already quite shitty, I don't have enough reason to trust them to not implement more post WW2 units into the game.
I personally could care less if it was never deployed as long as they had a prototype. My concern is the same as yours, once you let one random make believe unit in you let them all in and it is an easy way for relic to implement pay to win.
This thread is weird though as people want historically accurate but this game is not accurate in the slightest. Panthers should not be diving with their weak side armor, KV series should not be competing with late war tanks. Hell, right now the P4J is probably one of the best meds competing with the T3485 and E8. When it was first introduced it had horrible turret rotation to simulate german late war lack of resources. It was buffed since it sucked because balance. Similar situation with ISU152/IS2 stun mechanic, the gun wouldn't penetrate as often but the stun mechanic kept them in the fight allowing for support to come in and do their job. This game doesn't even attempt to simulate the upkeep of fielding heavy tanks, the closest was the Tiger ACE.