I really wanted the Ubuntu phone to succeed. I backed the Indiegogo for their fancy phone, and when that failed I installed Ubuntu OS on a Nexus 5 to play with.
I never activated any phone service on it but I think I would have enjoyed it if I had. It was kind of neat to have a smartphone that didn't hide the fact that it was a computer. Even without plugging it in to a monitor or anything, I was able to play with the Chrome dev tools on the fly and it was pretty fun.
I ran Ubuntu Touch briefly on a Nexus 5 as well. IIRC the two issues I had were:
1) calls and MMS not working well.
2) Instead of a normal GNU/Linux OS Ubuntu Touch tried really hard to have an Android style immutable OS. Kind of the worst of both worlds since you have a difficult to work with OS but without the app ecosystem of Android that some people believe makes it worth using.
After that I kind of just gave up on the idea that I owned my phone at all and a few years later I gave up carrying smartphones entirely.
I used to use Ubuntu Touch on my OnePlus One for quite a while and it was very nice! I had to switch away because it didn't really support group MMS (still doesn't from what I can tell), and then later US carriers started requiring VoLTE which it didn't support for a while either. But I still hope to switch back someday, that was the most enjoyable phone I'd used since the N900.
I haven't tried to use it full time for the same reason that I haven't tried to take the plunge to the dumb-phone world.
A part of me occasionally considers buying one of those cheap KaiOS dumbphones, because I figure that the stuff that's missing could be supplemented with my laptop. The problem I'm always afraid of is that I'm afraid that I'll be missing something. I'll start a job and they'll require some specific proprietary 2FA app, for example. Also, I don't think Signal has a dumb phone app yet and that's basically all I use to communicate.
Similarly, I haven't done anything but play with Ubuntu Touch, because I know there would be an app missing that I would need, so I'd be forced to have an Android or iPhone for those edge cases anyway.
Startup costs are enormous to break into the smartphone world now, at least if you want to penetrate into the "non-tech-enthusiast" market.
IMO it comes down to marketing: can't have the kayfabe of selling something that is "not a computer"/"new kind of computer" and have it act like a "computer" too
Marketing played a role yes, but plenty of other phone operating systems failed that had much stronger marketing then Ubuntu ever would have.
Not a phone, but I really liked the Blackberry Playbook's QNX system. It was extremely smooth and easy to use, and was fairly easy to develop for.
They shit the bed by betting on Flash, which was a dying tech, but I was pretty sad when Blackberry just went to "another Android phone", since I thought what they had was pretty neat.
Blackberry definitely had much better marketing than Ubuntu has ever had, and for reasons probably too complicated for me to fully understand, they're sort of a joke now.