My problem with Riot specifically and Matrix generally is that the adoption process isn't very gentle. Yes they have bridges, but my experience has been that they're way more pain than they're worth, especially if you're the odd one out.
The thing I like about Signal is that I can use it for regular texts as well as encrypted communications. Instead of converting all of my friends at once (or even just a significant subset), I can just replace my texting app with Signal and get my friends to move over one as a time.
I tried that with Riot, but it just didn't work out. I tried the Slack bridge, but it required admin access on the Slack server (fortunately I had that at the time) and only one channel could be created at a time, which was tedious. That's not Riot's fault, but it's not particularly important who is at fault.
I want to replace some app with Riot and slowly phase everything else in, but nothing really gave me the confidence to do so. I heard rumors that they were planning to support VoIP, but this doesn't seem to extend to making/receiving regular calls or texts, but merely providing audio and video chat along with their regular text service. Maybe that has changed (I check back periodically), but unless it solves a problem that doesn't require me to convince everyone else to switch, I'm probably not going to make the effort.
And that's why I like Signal. It's not the best secure messaging system out there, but it's a drop in replacement for something I use today and doesn't require me to get everyone to switch at once.