It has major reliability issues.

Any reliability issues is a major red flag for a protocol that's supposed to supplant SMS, which has no error detection or correction capability, and has a published message failure rate ~12%.

Just visit reddit and look at the kind of issues people have.

Above all else, it's still tied to your cell account. I already have an unreliable messenger tied to a cell account, what value is another one, that after 15 years still has reliability issues, and is dependent on support by a cell carrier (even worse for feature sets)? Being tied to my phone provides zero benefit to me (the opposite actually). So who does it benefit?

No other messaging system is reliant on carrier support/implementation. Why should any messenger be reliant on them, when we have a networking protocol designed to enable apps to be independent of the lower layers?

And before you say using your phone number makes you easy to find, first, other apps use the # as a way to find you, just look at WhatsApp, Telegram, Signal, second, how hard is it really to give someone your contact info? I'm "Bearofatime on lemm.ee". I've given my email and phone number to people verbally a million times, and if it's important, they send a message right then to ensure we're connected.

There are dozens of other messengers out there with zero reliance on carrier support, and work cross platform. They also provide the same features regardless of your carrier.

I was using XMPP on my phone in 2010, messaging people who were on their computers. No dependence on carrier support, no connection to my phone number. When I got to work, I'd login to the same messaging apps on my laptop.

XMPP blows RCS out of the water. So do many other messaging platforms (well, pretty much all of them). So RCS a solution in search of a problem. If it were available to me, why would I use it instead of the other messengers I already use?

As it is I already use multiple messengers, even for work we use 2 or 3, depending on the message.