For what it's worth my experience of just having made a large order of Warmaster Vampire Counts was a positive one. It all arrived, even though I wanted 8 or even 12 blister packs of certain unit types. I was reassured by the GW employee that whatever I ordered, as long as it was marked still available on the website, I would get the number I ordered as they would cast up more to fulfill the order? I was sceptical and said that was not my understanding of the state of things. They were true to their word and no units were missing due to no stock.
Could be worth a try, as long as you're after an army that has enough units left to choose from to make it worthwhile. E bay and other country's GW web sites are also an option to fill in gaps. Then again, you can get what you want from other manufacturors for many of the Warmaster armies. I got my ethereals from Kalistra!
Dave (cracked at the last minute and got that VC army!)