By bikers, for bikers by Bennetts,....... so make what you will of it. I would have thought it fairly standard advice not to ride or drive around on cracked or perished rubber valve stems at the best of times, irrespective of whether you had a gizmo attached or not. But that’s just me.
Can I buy metal stems for a TC GSA? I've Google'd the living daylights out of the subject but can't find straight metal valve stems?? I did ask Bahnstormer a while ago, they don't do them.
I have the FOBO Bike 2 system on my HD bikes on rubber valve stems and have done 10's of thousands of miles with no issue at all. They system works really well - and I quite like being able to check my tyre pressures while lying in bed!
I can't answer your question re rubber valve stems, as my Africa Twin has metal stems, but I do have the Michelin TPMS (it came as a freebie with my anakee adventures) and it's been spot on so far. Battery lasts ages, very useful to just see pressures at a glance.
I have been using these also for a couple of years, and with rubber valves. No problems at all, I now have them on both my bikes, I would recommend them.