It's just big enough that it might be barely usable to touch type with sufficient practice, not comfortably though. Definitely seems smaller than the Libretto from glancing at pictures. If the idea is to have a portable keyboard to then sit down somewhere and work, there are pocket BT ones that fold in half and are comfortably big.
The only use-case I can think for the Pinephone keyboard is if for some reason you want a physical keyboard to use while walking.
they discontinued it because of quality control reasons, which tracks with my experience. the key mechanism is weird, a circular membrane around plastic posts, and it makes some of the keys (especially the top row) constantly miss inputs. there's a 3D printed shim 'fix' but that barely helped me. off-center keypresses also bind really badly. the pogo pins that connect to the phone are flaky, which can make it lose connection and require a reboot. like the keys, there's a shim you can use (putting some paper behind the contacts) which only sort of helps.
a benefit of the keyboard case is the battery it has that extends the phone's battery, which is nice, but it's an odd setup too. the charge controller on it was originally designed for a power bank and seems like a bit of a crude solution that's lead to multiple issues, like how you could damage the phone having it plugged into USB while the keyboard case is connected.
it's slow to charge too, often it can't even keep up with the phone during use.
When your fingers are too fat and/or your hands are too big for a Libretto, or you have pathological issues troubling the relevant appendages, then do yourself a favor and don't bother.
It's obvious: These keyboards are, or should be, primarily made for mobile people, so high quality and tray or coffee table compatibility are a must. The Psion 5-series keyboards are ideal for this as long as you don't fuck with the formula.
My own opinion, I bought a Toshiba Libretto CT50 and the keyboard is really cool but also so small/hard to use.