I know there is a problem (or was) with Boinc accessing the GPU. when installed as a service. I assume this is still true even with 7.16.20 on Windows 10.

What if only the CPU is being used by Boinc? Will a service work ok then?

No reason why not. The restriction (no GPU in service mode) came about because of a Microsoft security restriction in the GPU driver model: I doubt they'll remove that now, even in Windows 11.

The big win is that service mode allows BOINC to do science when no user is logged into the console - ideal for servers.

No reason why not. The restriction (no GPU in service mode) came about because of a Microsoft security restriction in the GPU driver model: I doubt they'll remove that now, even in Windows 11.

The big win is that service mode allows BOINC to do science when no user is logged into the console - ideal for servers.

This worked out really well. The system I put the service on is headless, no keyboard, has no usable GPU (old HP 6000) but does have a core 2 quad. It is running rsync and is the backup for a NAS.

I set Boinc to use 3 cores and not run if %25 busy or if there is any keyboard / mouse activity.

When I logged in using remote desktop Boinc suspended the 3 WCG tasks. After 2 minutes of no activity the apps resumed.
When I started the backup app on the NAS Boinc immediately suspended the WCG apps. The apps started back up when the backup was done.

This worked out nice. It is unfortunate that a GPU cannot be used when it service mode. I wonder if that is a problem with the opencl or cuda implemenations or at the driver level (ATI, NVidia) or at the Microsoft driver handler.

I wonder if that is a problem with the opencl or cuda implemenations or at the driver level (ATI, NVidia) or at the Microsoft driver handler.

It's a feature, rather than a bug, in the Microsoft driver handler.

See WDDM Architecture: notice the dividing line between 'User Mode' and 'Kernel Mode'. Kernel mode applications, like services, can't access the bits of the driver above that line.