The problem is pretty much the same as it was in contact missions. That being everyone is given a shared pool of lives, and it punishes players for the mistakes others make. Heists took it a step further and made an already prevalent problem even worse by automatically failing missions if someone dies and there are no lives left, even if you are still alive and in the fight at the time. The BS excuse they give being the heist is impossible to continue without that person, which is ludicrous for most situations the five heists throw at you.
Demolition dies in Prison Break? Plane just has to evade missiles without any support from demolition, which is easily doable. One time I evaded a jet chasing me for ten minutes straight with no help from the demolition, and I did it easily. When everyone jumps out of the plane have them hop on motor bikes waiting for them on the shore instead and drive out.
Prisoner or Prison Guard die during Prison Break? The other simply keeps going without support from the other.
One of the ground team dies in Humane Labs? The other guy can complete it, just without any backup.
One of the defenders die Series A Funding? Just keep going, you got three other players there.
Someone dies during the escape out of Pacific Standard? Just have someone else pick up the bag of money they were carrying (if they were carrying any at all) and keep going. There is no vital role anyone plays during the escape.
Then to top it all off, if any player dies and the heist completed successfully, some of their cut is docked and distributed equally amongst the others.