When I was doing my HNC in horticulture, one of our lecturers was a scientist who had previously worked at a research station.
He told us that mycorrhizal treatments were of benefit on soil where there had been a change of use e.g. grassland or lawn converted to growing land or borders. There would be myorrhizal fungi in the grassland soil, but it would be the kind that links into grass roots. In the same way, it was useful when planting trees, unless you were planting in a woodland.
The general purpose packs contain some of everything, so if you can track down a specific type for a particular job, that could be a better bet. If the ground you are planting into has previously grown fruit and veg, it should not really be needed if the soil is in decent heart
