A related problem I had with those vouchers is the "not valid on UA-codeshare" rule.

With many of my flights originating in Europe, my intra-European feeder flight to the UA gateway (usually FRA) is a UA codeshare flight operated by LH.

Several UA domestic agents staunchly refused to ticket with the voucher. Recently UA Germany published a memo, specifically authorizing intra-EU codeshare to be ok with all other flights (transatlantic & North-American) as true UA flights.