SMS messages are text only, they are all very limited in size (kilobytes) and MMS messages are text plus embedded/attached graphics (a photo or video will always add more size so a MMS will typically be measured in megabytes). That's a given and it's always one or other, they are not interchangeable.
So that said, with SMS it only needs very low bandwidth transfer speeds, so just a base cellular signal is all that's required. No matter if you do or don't have WiFi or mobile data enabled, a SMS message can still be sent or received. With MMS however, because it does require more bandwidth to accommodate the larger file size transfer than it needs WiFi or mobile data.
A drawback is both SMS and MMS are very, very dated protocols. They are the only two protocols that most text messaging apps/services still support so they're the last two universally protocols that remain as a common link between all the text messaging services that compete with each other. The problem being they're so dated that they have limitations with today's current technology.
RCS is Google's attempt to compete with Apple's well-established iMessage. RCS is also a closed-source, proprietary text messaging protocol (managed by Google), and Google has spend a lot of resources trying to make RCS the Android default text messaging service, not unlike how Apple's platform is centered on its iMessage protocol. In a very generalized view, RCS is more a MMS replacement than a SMS. It's layered with more overhead so RCS file size transfers use more bandwidth (requiring WiFi or mobile data).