I must echo other replies here, never had any issues using LH vouchers, even got 2 used on the notorious SIN route very recently. We only get 2 of them per year (normal Plats) so hasn't been a big deal to use them with bit of planning in advance.

However, I can understand the possible frustration for someone required to travel long haul Y for work on a short notice when flights are often full (in J) already.