...lamp life will be a lot shorter.

Unless it is a disguised real 40W lamp...
But to distinguish that, you have to measure the filament...

They definitely start easier on weak or non Earthed ballasts.  I have a F40 two-tube ballast in an old light in the barn that will not start one of the tubes if a 40 watt is installed. It will start a 25 watt "UTSL" fine, and as bright as a F40.