The C5 that has been out for a year or so, or the new C7 which at this point is pure speculation if or when it will become available? There are sparse details about the C7 so I don't know if anyone could even begin to answer that question. Hubitat has been quiet about the subject of new hardware.
I'm not sure anyone has noticed specific differences between an external Z-Wave stick and the built-in radio of the C5. Staff recently removed the Schlage Z-Wave locks from the official compatibility list after a lengthy forum thread (and many in the past) where they concluded that some people were still having problems despite following the usual advice, and they aren't sure why. If anything, I might guess that the external Z-Wave stick would actually work better since you have more control over where it is placed. You can still use one of these on the C5 if you want--you'll just need a small adapter cable to make it able to connect (since there's only one port, a micro USB, for charging by default).
That being said, I'm not sure this would help. I'm sure you've read several threads here on the issue; if not, a search will find a few. Did you recently add your Z-Wave repeaters? If so, running a Z-Wave repair may help if you haven't tried that already. You may also need to wait a day or two for your mesh to "settle down" before drawing any conclusions. I'm also not sure if anyone has conclusively decided that battery-life problems were a mesh issue, but it's definitely a good idea to have a strong mesh full of beaming repeaters (check their Alliance certification, but anything certified within the last few years should be, if you aren't sure) if you have Z-Wave locks, regardless.
No need to be sad. They aren't removing the built-in driver or otherwise changing anything. If they work fine for you, they'll continue to work fine. As you're undoubtedly aware, there were just problems for some people, possibly related to old lock firmware versions or weak meshes, but they didn't for sure know the cause of every problem and this was probably easier than listening to people complain until one day they are hopefully more confident about how to make them work well for everyone.
I have two.
2018's Z-wave model (BE469 fw:MAIN_8.0) and this years Z-wave Plus model (BE469ZP fw: 0.10.9) .
The latest model has been great for me, the former has given me a ton of problems coming off of Wink (worked pretty well there), BUT, after range extenders and my z-wave mesh finally being settled (adding less devices now), I'm happy.
But I think the staff did the right thing here. Perhaps a caveat of others are in my boat could be to list the latest and greatest with a known good firmware as supported. Just a thought.
My lock was purchased over two years ago and has been solid. Sometimes my z-wave mesh acts up, but I do have a strong mesh as all of my switches are zwave and zwave plus. I have a couple of plugs that are plus repeaters that finish it off nicely. No troubles here. I had anticipated that I would have troubles, because of the age, but that is not so. I would never remove and try to join it again, though. If it's not broken, don't fix it!