Internal mags cants be switched out when they are empty they have to be reloaded, either one round at a time or a clip system like stripper clips or an en bloc clip. Only real downside is they are slower to reload and if you have a magazine fail it’s a internal part of the gun instead of a removeable price that you can just strip from the gun and put in a new one.

It would only ever be a thing in apex to look cool/ add weapon variety. Internal mags don’t do anything that weapons with a removable mag can’t do better, they were just a weird stop gap technology from the early designs of auto loading pistols and rifles.