Personally I do not have GPU cluster, but I had Expansion Backplane & still own splitter (both from the very same company =).
So, GPU limitations. The number of cards would most likely going to be defined by system You use. Most of mainstream boards might start having issues after 4-5 cards, a bit better would be able to handle 7-8 & server oriented units might be able to go over 10.. (take those numbers just a a rough guidelines to illustrate the picture =)
x1 lane as connection would limit the speed Your GPU could communicate with CPU & it very much depends what type of projects You're working. For some that is felt very much, while others do not bother about that speed drop at all.
One thing to mention, is that in certain situation You might notice some hangs or crashes due to x1 interface (we have a thread in this forum somewhere & Guys do share their insights about that problem - wasn't following that lately so honestly do not know how it goes now - will post a link to thread if I find it).
Overall it's great product that might help You to expand Your rig easily, but You get what You pay & by that I mean.. this solution for 4x GPUs on x1 interface costs like 450$ while on x8 or x16 would cost You over 2k$.. whether You would be happy with it or not much more depends on Your needs & preferences rather than anything else (at least that's my opinion =)