HBM2 wasn't used because it's terribly expensive and would have prevented AMD from hitting their price target.
If the 5700XT is performing worse than it should, it's probably due to the new arch. The same has happened with NV in the past. Requires the projects to update their code. It should perform only about 12% worse than the VII at FP32 but capable of only 20% of the FP64 the VII can do.