I unlocked my 14 Pro with their service today as well, but it took quite some time due to the servers apparently being in maintenance,
As they are using teamviewer, which is known to be a huge security risk, I opted to use a sandboxed Windows and tracked what they were doing locally.
In my case they installed Radmin VPN, an USB redirector tool (which proxies the USB slot to their computer), and the adb & fastboot drivers.
They didn't try to anything funny in obvious nor stealthy ways, so all good on that front.
Granted, I would nonetheless recommend to use a fresh Windows installation while at least removing the drive letters to your other partitions, simply to not take any risks and to not pollute your main Windows with TeamViewer (stays a security risk) and a VPN you will likely never use yourself.
Besides that,
the unlock process they use is obviously different from the normal consumer one. They don't use the normal unlock server but one for technicians/repair services. It also looks like they also trigger a maintenance mode somehow, as when I was prompted to remove my MI account, I could do so without entering my MI password, which is usually necessary. Overall it seems to me like they use the repair procedures that open up access to the whole phone to unlock the bootloader for the customer, instead of relocking it again like a normal repair shop would do after a repair.