For me, on a mid-2010 15" MacBook Pro, OCLP-installed Monterey 12.5.1 works well for web-browsing (Firefox) and email (built-in Mail app), but other apps are problematic. The Maps app doesn't actually display maps, for example, and the Podcasts app is barely usable due to settings controls being either unresponsive or difficult to operate (you have to use Accessibility key combinations). I suspect that a lot of the other built-in apps will have the same problems, but I haven't tried them, nor have I tried FileVault. It's my understanding that these problems stem from not having a metal-capable GPU, and if you have a 2012 or newer metal-capable machine things should work much better.How about the Apple Maps and FileVault? Does it work well under Big Sur or Monterey on you unsupported Mac?
OCLP provides more security than older patchers, which in itself can be a problem. SIP is partially turned on, and you can't turn it off. They have a library validation patch that keeps some things from working (iTunes installed with Retroactive, for example). If you install other versions of macOS along with OCLP, SIP will be partially on for them too.
On the plus side, OCLP-installed Monterey and Big Sur are still getting security updates, and the updates can be installed "over the air." But, the updates are huge, pretty much as large as an install app, so you might as well download the latest installer and make a new USB.
The OCLP developers have done a remarkable job, and it seems to be getting better all the time. But you need to temper your expectations.