At least on T-Mobile most unlocked phones do get WiFi calling and VoLTE. T-Mobile is not like AT&T where these features aren't available on unlocked phones. Sure there are few unlocked phones that don't have WiFi calling and VoLTE on T-Mobile like for example latest Sony XZ2 flagship but that's more of Sony fault and don't think TMobile is doing anything to block it. Heck even OnePlus 6 has VoLTE and WiFi calling on T-Mobile.
Overall I have been really happy buying using non carrier unlocked phones on T-Mobile. Been a while since I used a carrier crapped up branded phone and couldn't be more happier.
As for as sales, yeah it's going to hurt not being on carrier stores since people only buy carrier branded phones. Unless you are Samsung, carriers don't work with anyone else very well in the US. With Apple they don't have a choice, Apple treats the carriers like **** but the carriers have no power when it comes to Apple and have to carry them. For other Android OEMs they don't have the budget and market presence to push carriers (chicken and egg problem). At least in US it's a done deal between Samsung and Apple for majority sales. The niche knowledgeable market which is relatively small will probably shop around and willing to buy outside of carriers.
At least the U12+ is on Amazon, who have great customer service. I have placed my pre order from them, let's see how it goes once I've receive it in June.
PS: Yes there is LG, Motorola etc who have carrier relations in US. But they don't seem that great compared to Samsung, Apple.