Sorry, Bob, I am not a PowerShell expert.
I can tell you this SFXCL syntax is valid, but you already know that from your Windows command shell test:
Code:
SFXCL /Q /Log "some log file path" /I "path to key file" /P "passphrase" /F "path to alternate config" \\fileserver\sourcefile.txt sftp://user@host:/someremotedirectory
It's likely you could have missed escaping some special character.
Are you getting an error?
If so, what is it?
Troubleshooting PowerShell is outside the scope of SecureFX support, but if you post it, maybe the user community can help.