As mentioned in class, the advantage of HBM on chips is that it is orders of magnitude faster than normal external DRAM without a similar increase in power consumption. The disadvantage is that it isn't as practical as normal DRAM. Not only can you not add/replace the memory after you buy the chip, but often HBM chips offer a lower capacity than external DRAM.

Adding to @tp, it was also mentioned in class that stacking DRAMs to have HBM is a tradeoff to capacity/memory space. This means we give up the size of our memory in exchange for HBM.