Always had a huge lorry or trailer load of mushroom compost every year. Mushroom farms locally until about 4 yeas ago. Perfect for veg patch. Applied it thick...6" or more......in the autumn and in spring I cultivated it into the soil surface. Produced superb crops without weeds. Also used it as thick mulch around everything ESP hellebores. Except couple of camellias I grow. Azaleas, pieris. Rhodos etc grow in large pots and kept away from mushroom compost.
Mushroom farms not so prolific. Now amd then I get load of manure too as well as recycled compost from the council.
High in lime but low in nutrients. To allow for any chemicals in it......always bought as organic mushroom compost......I apply in autumn and allow the winter rains to wash it through the soil so that come the spring the compost is "clean"