Is update preview Alpha stable at all?

Seems a long time to wait. From Beta to Omega, and then GA, just to get a OSU1 file thats older than Beta preview the console is on. A Factory reset is quicker than waiting for a file that may not solve a potential hardware issue.

"on my sons console that is not in the beta but has just updated to the may dashboard,".

As far as I'm aware the new dashboard is only available to Alpha and Alpha Skip Ahead previews.

Edit (22-5-2023) A subset of BETA Insider previews have the new Dashboard

"so just like the previous times, once the osu file is higher than what the console is asking for it will update with no issues."

I would investigate your network access if you need to use a OFFLINE System update file (OSU1) all the time. A stable network is required for all the preview updates that Alpha and Skip Ahead receive each week.

Depending on preview, a Insider console is updated several times over the course of preview.

ASA preview is especially demanding of updates, and if one or two are missed, well, lets just say its not convenient.

This has supposedly been fixed, along with a bunch of other stuff, in an update that's available for people in the preview program (alpha ring) now.

So, if you're in the alpha ring, check for updates and you should see it there ready to install.

If you're not in the alpha ring, or you're not in the preview program at all, I expect they'll roll out an update with a bunch of fixes in it to the public fairly soon.

As an Insiders You gotta make a report for all the bugs You see, You must be in Alpha Ring, this ring is the most unstable, but not more than Alpha Skip Ahead, when You decided to join the Insider Program You knew there were a lot of bugs in the preview
OS, You can change another Ring, Beta, Delta or Omega, but the most stable of them is the Omega ring, now it's You who decid which ring You Will choose to recieve updates or If You Will choose to recieve updates like a normal user.

If your console is in the Insider Preview (Alpha or Alpha Skip ahead) try to disconnect all controllers and headsets, and only connect one controller and one headset.

Also read here: https://news.xbox.com/en-us/2018/07/11/new-preview-alpha-1810-update-7-11-18/

Are you in the Xbox Early Access (update preview) programme? These are all problems associated with those updates while in preview, but shouldn't have been issues when on the public (stable) releases.