magnets come sizes... shapes... strengths
there is more to a magnet than the magnet. there is how it interacts with other magnets, the strings and metal in the pickup
different magnets can be shaped (ceramic is easy - its molded and fired), drilled, threaded, cut, etc etc etc into shapes.
the shape and strength and interaction determine the "magnetic circuit" or how the lines of magnetic force flow from magnet, pole, strings, to other end of magnet and through the coil
BL has used all sorts of magnets in his pickup designs - the magnet type matters not. what matters is the magnetic circuit - what the designer wants the magnet to do in terms of the circuit, and its interaction with the strings and coil - and then find a magnet of some type that will adhere to the form factor and be the correct strength (too strong is no too too)
FWIW, Fender cautions against raising alnico V magnets too close to strings - creates nasty harmonics from magnetic pull on strings - such is a "too strong" magnet