Stronger language is warranted for hash indexes.
In a simple test, there was an order of magnitude difference in load and GII performance between the two types of indexes.
Client: Loaded 2,000,000 entries in 20,487 ms
Server 1 Size: 1,272,322,680 bytes
Server 2: GIIed in 12,523 ms
Server 2 Size: 1,272,088,544 bytes
Client: Loaded 2,000,000 entries in 257,016 ms
Server 1 Size: 1,228,010,280 bytes
Server 2: GIIed in 218,983 ms
Server 2 Size: 1,228,161,664 bytes
So, the difference in size was ~45MB, but the difference in time was an order of magnitude.
Better wording suggested for file developing/query_index/creating_hash_indexes.html:
Using hash index will degrade put performance and recovery time substantially. If memory is not a concern, it is recommended that you use range index.