Perhaps your users don't have the same abilities with ssh?
The reason why xpra switched to paramiko as default ssh implementation is because it makes it possible to integrate (cross platform too) with the GUI so that asking for passphrases or passwords can be handled by the xpra process itself (it may already have the password or it may delegate to pinentry or gpg agent or putty agent or whatever).
This also means that ssh errors can be handled much more gracefully - natively in the code.
With openssh launched as a subprocess, the user interface is non-existent, and when the ssh process fails, all the xpra process sees is a dead process with a non-zero exit code - which is much more difficult to handle gracefully.