Good god, mandrakes. That statline is typical DE, but you get four fucking strength!
^Yeah, maybe that's why.
What I like to do is infiltrate them, and have a homonculous in a venom(Night fields ofcourse) scoot up 12", disembark near the mandrakes, move the mandrakes up so that they're within 2", and bam. Pain token number one. Now, you won't get any assaults, but you will get some delicious S4 AP4 pinning shots. 18" assault 2 guns will deliver Dire Avengers to Craftworld Cowardice in no time flat. Not only are you going to most likely cause a pinning test, even against MEQs, but against everything else? Morale test too. And you've got FNP on your drakes, so if you get shot back, you got a 5++/4+ to hide behind. Stick them in cover for some very funny 3+/4+ saves, and no one will even bother shooting, half the time. When you get to an assault, their 2A base, 5 initiative, and 5+ invuln come in real handy. You can tarpit an IC for a pretty good amount of time with them, too. Shrug off S10 powerfists all day long pretty much.
With shooting, give your homonculous a Casket of Flensing for some added lulz. I've never had luck with it, but you can potentially get an Assault 12, S6 AP1 gun. It's only like 10 points, so why not? You'll be getting another pain token pretty quick if you can roll well with that. If you paid the points for an Ancient, they're hitting on 2s, also.
Worst case scenario(The one I usually get), you get a S1 AP6 assault 5 or 6 gun, but can't really do much with it. The Mandrake's shooting however is very constant. Hitting on 3s all day, wounding MEQs on 4s. Thing is, every troop that has a 4+ save, always has T3 also. So when you're shooting against prime targets, you'll hit on 3s and wound on 3s also. Then they don't get any saves, and go crying for their mothers.
Five Mandrakes is 75 points, and a Nightfiend is only 10 more. 85 points for these little guys is a good deal, but they don't really scale up around 1500+ points or more, since that little 85pt unit takes up a WHOLE elites slot. You can make them bigger, but they go down easy to small arms fire unless infiltrated properly, which you have to make SURE to do. Even if you get the first turn, if your enemy seizes and they're sitting out in the open - say goodbye.
The homonculous+drake combo is real neat, but if you are taking webway portals - even better! Drop that webway portal out so that you can block off other potential shooters from line of sight towards your drakes. Next turn, you have a safe entry way for your reserves too, with your drakes at the foothill, or tying up an enemy unit. With move through cover AND fleet, they can get from that webway portal, and tie up a tac squad really easy. Just detach the homonculous from them, give them the pain token, and they'll last in combat. Since they're I5, they go off before almost all of their prey, which makes the retaliation less harsh since you only typically have 5 members.
Try them out!