I have (iOS-using) friends I have convinced to register their numbers with Signal...but they never check Signal. They do check SMS.

This actually drove me away from using Signal for SMS on Android, because I couldn't send those friends SMS messages through Signal. They could only go out as Signal messages, and my friends never checked them.

Fortunately, I only have a handful of friends in this category, and virtually everyone else I talk to does use and check Signal, but it's a usability problem both ways.