Its been a flawless experience and I'll never look back.
From a technical perspective I think it could be easy to believe that because they're a startup your money is in someway less safe, or the service less reliable.
But I actually think that because of the sheer technical debt old banks struggle with (they're ticking time bombs on the inside) the opposite might be true.
Personally I switched almost a year ago (when they introduced CA), and am happy with that choice so far - kept my old bank just in case but haven't needed it. Re outages many banks have had major outages recently, far more than Monzo... They had a few glitches early on (late last year), but I'm fairly confident of their ability to scale now.
However it is a feature that Monzo had, which was easy to use and they got rid of it (IIRC due to the costs of providing it), which decreased usability, which I think was the original point the poster made...
This provided a nice usuable way to get money into the account from a "legacy" bank that doesn't have a good mobile story.
Now if you have Monzo + a legacy account (as many do) you have to go log-in to the legacy account to move money to Monzo.
Ofc once you go "full monzo" that's not a problem any more but it's still early days for that and Monzo have had enough service glitches to make people a little nervous of committing to having it as their only account, I think.