Another possible disadvantage (although I don't think that I've seen any specific reports about this one way or the other) is the possibility that if your voucher is on an FTV, you will only be able to make a new booking for the companion who was on the original booking.

It would be good to have confirmation one way or the other about this.

One other thing that I'm not clear about is whether, if a voucher is held on an FTV, it can be fully cancelled back into the voucher holder's account; or whether it can be used to rebook and then the new booking fully cancelled for a refund.

That's also linked to the other query in my mind, which is whether an FTV that holds (say) 10,000 Avios + £17.50 can be cancelled for a 10,000 and zero cash refund (to mirror the normal position on the cancellation of an Avios booking where less than £35.00 cash has been paid), or whether this requires payment of the full £35.00 even though the original Avios booking only involved a smaller sum of cash.