Earthworm castings have earned their reputation honestly, but the reputation is usually described in a way that leads people to use them wrongly. They are not a strong fertiliser and they are not a growing medium. They are an organic amendment with a genuine effect on how a mix behaves, and their real value is at the margin rather than the bulk.
What castings actually do
The nutrients in castings are present in modest amounts and in slowly available form, which is why they cannot burn a root the way a concentrated mineral feed can. The bigger effect is physical and biological: a mix dressed with castings holds moisture more evenly, stays open, and carries a much larger population of active micro-organisms. Seed and cutting composts benefit from that; a pot that is already compacted does not, because the drainage problem was structural.
That distinction is the whole argument about how to use them. Anything you would want to achieve through a chemical drench is not what castings deliver.
Two ways to apply them, both correct
Mixed in: roughly one part castings to one part soil for a potting mix, blended through rather than layered. At that rate the mix gains the water-holding and microbial behaviour without losing its structure, and it is the version worth doing when potting up anything that will live in the container for more than a season.
Top-dressed: a thin layer, an inch or so, spread over the surface of an established pot and then watered through so the material works down into the upper roots. This is the low-effort version and it suits plants you are not repotting this year. Do not scratch it deep into the surface of a shallow-rooted plant.
What is not worth doing is filling a pot with pure castings. It is expensive, and it changes a variable that was not the limiting factor: a 100% casting pot loses its open structure once the material settles and goes compact and waterlogged. The same reasoning applies to every rich organic amendment: they belong in a mix, not as the mix. That is the argument set out at greater length in the comparison of organic with inorganic fertiliser.
Where they are a poor buy
Skip castings for anything that wants lean, gritty conditions. Succulents, cacti, rosemary and the Mediterranean herbs do better in a mix with grit and low fertility, and adding a soft, humus-loaded material to their pot reintroduces exactly the water-retention problem that kills them. For those plants the castings are best spent elsewhere, and the same is true of seedlings already in a sterile sowing compost, where the microbial load is not a benefit you need.
If you are making your own mix from homemade compost or dug garden soil, treat that material before it goes into the pot rather than dressing it afterwards. How to disinfect garden soil and homemade compost covers the methods that work, and castings added to an unsterilised mix do not compensate for whatever is in it.