That's a really subjective question, so keep that in mind as I share my opinions on this. I have found PCFs to be very reliable for long-term supportability - more so than formscript, for sure. I think they're probably not as ironclad as a plugin (there are definitely plugins that have been happily cranking along untouched for a decade in many customer implementations), but that is simply the nature of the rapid evolution of front-ends as opposed to the relatively static design of data and event structures.
@DianaBirkelbach and I answer a lot of PCF questions on this forum. In the last year or so, I have probably only seen a handful of "my PCF was working before, now it's broken!" cases. Usually, that is to do with someone accessing non-public object attributes that were left out of the typescript classes specifically because MSFT intended to change them (just because you can find an object in your browser dev tools doesn't mean MSFT wants you to use it!).
Generally, as long as you only use supported mechanisms, your PCF should be rock steady; I think I have only seen one case in the last year where a fully supported approach stopped working 'suddenly', and it was really because there was an underlying issue and the approach was not tested as well as it should have been.
But--of course--none of this means your PCF will be bulletproof. MSFT could put out changes that break your PCF at any time; that's simply the nature of developing on a platform like this (in particular with a comparatively young toolset like PCF). So, if you need to distribute the solution to many consumers, you write the best, safest code you can, then you plan for the worst and figure out how you're going to distribute hotfixes if a future release breaks you. There isn't much else you can do.
