I use Mint which was an MVNO for T-Mobile. They've since been acquired by T-Mobile, but I haven't really experienced any difference in service. I also haven't used Visible, so I can't directly speak to that but Visible is owned by Verizon FWIW.

I probably experience times with deprioritization. 5G service can go from getting several hundred megabits with pretty low latencies to only getting a few megabits with potentially up to 100ms or so latency, depending on crowds. I don't recall any times where I had good signal but couldn't get any data, but definitely been in places where it'll struggle to do video chats or something at a big live event that doesn't have the extra 5G infra deployed. For example, an extra large crowd at the park for some event will probably give poor network experience but I'll otherwise get good connectivity in a modern sports arena.

In the end it's just a value proposition. Is having really fast network everywhere, all the time really that worth it to you? For many the answer is yes. But for me, on my personal device, if I'm getting poor data rates that's probably a clue I should really be putting my phone down and get back into whatever is happening in the park so I don't mind and the savings are quite nice.