I researched this a number of years ago. I found a very old article discussing their use on a golf course that had flooding issues.
After reading the article I was reassured that especially in clay soils, spent hops were the way to go.
My clay soil defied all previous attempts at redemption, in the summer the spade would bounce off and a fork would bend / snap. In the winter it was just a slippy, slimey mess.
Since using a combination including hops the whole thing has turned around. I can dig the whole veg plot in an afternoon (not a huge plot but previously unworkable). I can now push my hand / arm directly into the soil up to my elbow without a problem, parsnips are amazing!
I use it as a surface mulch 4 " thick over the winter and dig any remnants in in the spring.
My concern was that like beer slug traps I might be making another problem worse! This hasn't happened at all.
Chuck in the compost and coffee grounds as you go.
I also used sawdust - yes I know BUT used sparingly and soaked in wee (the nitrogen in the wee feeds the bugs that initiate breakdown of the cellulose and hence there's no nitrogen robbing action) it helps to break up the soil, retain moisture and encourage fungal colonisation of the soil.
No down sides in 10 years of doing this as far as I can see.