Death from Above is probably the hardest but if you are with 2 or 3 decent players (a rarity, I know) who don't go charging in all guns blazing it's not really too bad if you know who use the trigger happy and blood-thirsty cops to your advantage.
More often or not it's the players that make the mission difficult by rushing in and wasting team lives. I've seen it countless times. Team members go rushing in with a know it all attitude and get wasted whilst I tend to think about the mission, take my time, go for the objective and succeed.
Take the mission where you have to steal a Lost van for Trevor. Easy mission but I've seen so many players go charging in and last seconds because they try to kill everyone in sight - pointless as the enemies will just respawn. I'll go in stealthy. Round the outside of the trailers, jump the fence, shotgun the 2 or 3 guys at the first van, steal the van, on the train tracks, ignore any enemies shooting from the road and you're home dry. The same can be said for most missions, it's the wrong tactics and total lack of organisation that makes it hard. This is why so many heists will fail.
I did Landing Gear yesterday with 3 other guys yesterday. On the first attempt we were just bantering on the headsets. After that failure we came up with a strategy and only failed when trying to land the planes at the harbour.