Putting Ubuntu on an older MacBook Pro, any gotchas?

The Wi-Fi is the main one. Most of those MacBooks have a Broadcom chip that needs a driver Ubuntu doesn't ship by default, so have ethernet or USB tethering from your phone during install, then enable the driver in Additional Drivers. Hold Option at startup to pick the USB stick. If it's a 2018 or later model with the T2 chip it's a lot more work, check the year first.