I personally have had a very different experience, at least on the technology side, with RingCentral.
We switched from our on-prem IPO to RingCentral for our company five years ago and it has been great. Call quality is always perfect if you have good internet and we experienced a single one hour voice outage in that entire five years. The desk phones work fine. The apps are the best in the industry in my opinion. They are constantly adding good features with AI, etc. I have about 50 customers ranging from one user to a thousand users on RingCentral and I have never had any cancel except for one lawyer who retired. They are all happy with the service.
One caveat - you need to learn the programming yourself. Their support is overseas and bad. Their "professional services" is bad. It is a very intuitive system.
I have a few customers using ACO. It works the same because it is the same for the most part, and shockingly the Avaya support has improved drastically over the past few years and is now better than RingCentral's support.
There ARE things you can do on IPO that you cannot do on RingCentral. Like buttons on physical phones to log in and out of hunt groups, set night service, getting granular with shortcodes for restricting calls, etc. But for the uptime and app quality, I think it can't be beaten.