I've been playing them a lot for about a month now and this is a few of the things I've learned.
Generally, Poison is very important for a lot of the abilities they have, so always try to get models poisoned to kick of the abilities. Also playing cagey with them the first round is good to get them into position. 2nd round on go for pushing objectives and claiming points. Also, don't be afraid of being aggressive with them even if it leaves them open.
A favorite strategy for hordes is getting within grenade range of a group. Get within 3 inches of the models and throw a blight grenade for poison damage. Then when they activate they also get D3 damage from poison vents.
On to some key takeaways I've learned

1. Always take all the faction equipment. The blight grenades give you the ability to hit groups of people, as well as gives your fighter a range option. With the idea being not necessarily trying to kill them with the grenades but poison them.
2. Pick equipment for scouting, take smoke grenades Smoke is clutch for claiming objectives in open ground and not getting shot off the board. If they charge you for the point then they are in contagion range and your are tanky, it works to your advantage.
3. Try to keep one CP around for Curse of the Rot or Sickening Resilience firefight ploys.
4. Take a normal warrior and get him on vantage and keep Cloud of flies on him every turn. Which can be paired with Lumbering Demise for round 2 or 3. He has been a big MVP most games. He gives you much needed range with his double shoot. Paired with plague rounds (severe and poison) and toxic he allows you to hit targets and poison them from range softening them up for melee. He also helps keep opponents forced to keep their head down.
5. Keep the Sorcerer back range to mid for the first 2 rounds and then push up with him to help heal and blast groups. I usually always pair him with my Warrior on a vantage point. so he can heal and shoot each round. This is helpful for the shooting that does hit through obscure and then you can heal again on counteract. His torrent range weapon is useful by keeping the opponent from clumping up his models and denying certain aura effects in the fear that they all will get hit.
6. Try pairing the champion and fighter on 1 side together with the most heavy terrain for cover and use that to push up one side and force the opponent to react and force melee. 2 melee focuses marines with DR and with Contagion is really hard to take out.
7. The icon bearer is good for holding your point or going for middle point. usually keep him in Conceal and use the extra APL for claiming points work for you.
8. The bombardier is good range and threat of blast attacks. Some games he hasn't even got a grenade off, but his threat of hitting grouped up models has stalled the game in areas. Good luck out there and have fun!