First off, putting this in support because I'm not sure if it's a 1.8 thing but not sure if it's a a bug either.
So, onto the topic. I have set up my own server that I play with my brother with IC2 1.15, Buildcraft 2.2.1, the IC2 addon Advanced Machines, and Minefactory Reloaded 1.2.2. (I also have Rei's minimap installed client-side, but I doubt it's the problem). So my brother and I have been playing for a day or two, and we have not encountered one diamond. (and we have made huge amounts of layer 16 diamond shafts, and explored many layer 1-16 caves. We have also encountered barely any redstone, which again I'm not sure if it's a 1.8 thing. However, what has pointed me towards IC2, is that we are encountering MASSIVE amounts of copper. I'm not sure if this is 1.8 or IC2 related so any help would be appreciated.
EDIT: Also not sure is this is an SMP issue, as in IC2 1.00 in singleplayer I had no problem. Will test later.