A Pi running ScreenlyOS shows content at up to 1080p, and behaves the same as a Screenly Player in almost every respect. It updates itself, reports its health, and is managed entirely from your dashboard with no login on the device.
Where the limits are
For 4K, or for the TPM and Secure Boot that stricter environments ask for, you need the Screenly Player Max instead. Those are properties of the hardware, so no software setting adds them to a Pi.
Driving two displays as one joined surface is confirmed on the Player Max and on a Scala Player-SP. We have not confirmed it on a Raspberry Pi, so treat a Pi as a single-display device unless you have talked to us.
Everything else that makes ScreenlyOS what it is applies here: the device runs nothing but Screenly, updates apply and roll back on their own, and there is no shell to log into.
A Raspberry Pi running our software is not a Screenly Player. We use that name for hardware we sell, which matters when you are reading hardware-specific pages.