Hmm, there could also be the problem if it doesnt kill them instantly, it could boost them INCREDIBLY high and basically let them have their way with your base/homing in on the flag....
1. The only practical place that i'd see landmine placement would be extremely tight spaces...fair
2. On health and grenade boxes...somewhat unfair
3. behind boxes...fair...stupid box campers
4. On your team flag...extremely unfair and most often than not, would be done so by the teams