Mushroom compost (SMS, spent mushroom substrate) in my opinion would be the last resort source of humus. Look for the finest quality EWC and you'll be a very happy gardener.
I have used it though quite indirectly. I was given a bag of it once and tossed it into my compost heap and then ultimately it went through my worm bin. Directly I have never used it nor do I plan to.
Mushroom Compost "What beneficial properties are found in mushroom compost? The visual appearance of a good quality, thoroughly processed mushroom compost typically resembles a dark topsoil, has a loose crumbly structure, and has an “earthy” aroma. Recent research conducted at the Pennsylvania State University showed that mushroom compost contains an average of 25 percent organic matter and 58 percent moisture on a wet volume basis. Where uniform application and good mixing with soil is required, this amount of organic matter and moisture in mushroom compost is ideal for handling and making surface applications or incorporating into the soil. Mushroom compost contains an average of 1.12 percent nitrogen in a mostly organic form that slowly is available to plants. Also, mushroom compost contains an average of 0.67 percent phosphate (phosphorous) and 1.24 percent potash (potassium), as well as other plant nutrients such as calcium (2.29 percent) magnesium (0.35 percent) and iron (1.07 percent). The average pH of mushroom compost is 6.6 (6.0 to 7.0 is an ideal range for most plants). The amount of carbon relative to nitrogen is an important indicator of nitrogen availability for plant growth, and an ideal compost should have a ratio of 30:1 or lower. Mushroom compost has an excellent 13:1 ratio, indicating outstanding nutrient availability and mature and stable organic compost." I've used it but I use it with a variety of other amendments. I mix it with other types of humus and EWC. It is cheap and readily available at Lowes or Home Depot so I figure why not.
Mushroom compost, sms, or spent mushroom substrate is what I would consider the worst choice of compost to use right after unfinished compost. The reason why it's called "spent" mushroom substrate is because that's exactly what it it. It was once good quality organic matter used by mushroom faming, heated and treated with salts to kill all other microbes (even beneficial ones) to make way for particular fungi to be inoculated and dominate the "substrate". The fungi consume the organic matter, grow into full mushrooms, mushrooms are then harvested, and the spent substrate is then thrown away. Or in this case repackaged as "mushroom compost" which sounds fantastic but really isn't. I guess this product truly does show that "one mans trash is another mans treasure". It just depends on which man you are, to see it as trash or treasure. Do yourself a favor and find some real quality humus. And except nothing less.
I have used it once directly, a mistake I'll not repeat. Got 2 bags for cheap at HD and the second bag is still in my garage 2+ years later. Wet