Hey mate, so I am running Changehost as well and full disclaimer, I haven't played a battle with it as of yet as I'm still building and painting it, however I have done a lot of research so would like to just share what I've read, but yeah take it or leave it man.
FEC just have crazy movement so will be up in your face right away so it can be hard to screen against them if you take second turn (this is in general and not just for Tzeentchy bois) and also their Zombie Dragon can end up doing more Damage than most big hero's from memory (think I read somewhere on TGA that it can do even more Damage than Archaon? but I can't find the link to confirm this).
Skaven are very similar to FEC and can be hard to counter, but the Changehost ability can really help with this in terms of screening and wiping them out with spells.
From others and their posts on various forums etc, FEC, Skaven and Khorne can be some of our worst match ups really. Not sure if you have played against Khorne before, but they have this disgusting thing where their Hexgore Skulls subtract 2 from casting if a model is WITHIN 12" of either of the models (Hexgores have 2 skulls which need to remain wholly within 6" of each other so can cover a lot of ground) then if the unmodified casting roll for a Wizard is an 8 then the spell is unsuccessful, the Wizard no longer knows the spell and each Wizard within 12" of the Hexgore Skull suffers D6 Mortal wounds... this is a Wizard KILLER and my advice is to always snipe / take out their Slaughter Priests (the guys who are able to bring these onto the board) as quickly as you can with spells. Khorne really hates Magic.
I honestly feel that the best option for CH is to really screen your enemy correctly, explode those pinks and bog them down whilst smashing them with the endless spells!
My advice is to include a Gaunt Summoner into your list - his spell Infernal Flames is actually invaluable against Hordes, give it a read, then calculate the damage you can do to 100 Rats (which Skaven can take quite easily).
Anyways, I plan on posting my thoughts on Changehost when I have it fully built and test it against a few different enemies! Hope this helps mate.