How does the app compare to something like Electrum in terms of more advanced functionality - having manual control over which UTXOs to spend from, sending to many, freezing coins/addresses, etc? Is is possible to use a passphrase with this wallet?
You can set up a "Passcode account" which is basically a passhprase. It looks like you can have one such account at a time. There's some kind of multisig implemented but it forces me to set an avatar

and enter another person's UID (assigned by the Bitpie app). I was about to fund the wallet with a small amount of coins so I will get back to you once I have tested sending transactions.
The device itself looks very sleek, and I like the design, but if I can't use it with Electrum and their app doesn't provide all the functions I regularly use, then it's going to be a no from me.
The device officially supports only Android and iOS. I wouldn't expect
PC support anytime soon.