Vega10 (11.5+ TFLOPs) has more compute potential than 1080 Ti (10.6+ TFLOPs).
Vega10 has HBCC which adds some latency that was intended to be compensated with massive memory bandwidth which Vega10 did not have. Vega20 finally delievers both.
All the bandwidth in the world will not hide latency, nor is latency the true problem for GCN. The problem is simply resource management. The problem is not new with Vega either, it also existed in Polaris and Fiji.
If you have a GPU like GP104 with a 256-bit GDDR controller, it's really four separate 64-bit, supplying a total of 320 GB/s of theoretical bandwidth, but only when load is spread evenly across them. Each controller can only be used by one cluster at the time, meaning if four clusters are scheduled to read/write from the same memory controller, they have to wait in turn, leaving you with an effective 80 GB/s instead of 320 GB/s.
The reason why Nvidia scales better is they manage their resources much better, while AMD have a much simpler and more brute-force approach throwing much more resources at the problem, resulting in much larger and more power-hungry designs. Simply adding more resources (memory bandwidth, higher clocks, more cores, etc.) is not necessarily going to solve any problems if they are going to manage the extra resources just as poorly. More bandwidth is not going to solve any bottlenecks caused by data dependencies. If AMD threw more memory bandwidth into their design, they might risk decreasing the energy efficiency.