I dont think its an actual issue, its just the cost of the increased latency of bigger chips. Thats why you will have anywhere from 4 to 32 core Zen based chips. What we have to wait and see is if the GB4 scaling factor on Xeon is the same for Zen.
we have 3 data points so far.
1. GB4 32C 2P ST score, per clock around the same perf as 22 core E5v4
2. GB4 32C 2P MT score, something is wrong, if i had to guess i would say memory config, for 128gb across 16 memory controllers would be 8gb dimms, no one buys 8gb dimms. OR Zen could jsut suck
3. AMD "canned" demo of 8 core Zen matching Broadwell-E clock for clock in blender.
Its not a lot to go on, but compare that relative to the first bulldozer benchmarks.................