Been doing heavy power and RF for the last twenty years, not overly convinced about the whole EM field danger thing but bear in mind we evolved to cope with the background radiation levels and natural EM from the sun, we have not evolved yet to cope with the man made environment so to be on the safe side would avoid a house next to a substation or radio mast.
I think more of the problem is from oil/chemical leakage from the site itself (they are supposed to be sealed and enclosed but….old transformers will always leak and old transformer oil will probably have had PCB’s or other lovelies in it, if its from the 70’s or earlier (believe me they are still about as a 400kv grid transformer is not cheap to replace).
Also “you tube” sub station explosion, a nearby lightning strike overload can lead to a fireball and/or explosion.
A 1MVA ground mount transformer will contain a lot of flammable oil.