My understanding is that HUMS is mandatory in the UK on all oil and gas helicopters as directed by the CAA in CAP753?
In Europe, I don't believe there is any such legal requirement to have HUMS fitted, however in general the majority do as it's a customer requirement (not a legal one) I don't think EASA have made it mandatory but I may be wrong?
We don't hear about the 'HUMS successes' we generally only hear of the failures.
But yes, you're correct, if it is a transmission related failure and it's proven that VHM could have caught it, then you never know, they may introduce it as a mandatory requirement in EMS too.