Only doing it as a hobby, I don’t build forks at the moment for various reasons – the main one being as Ben describes above (and the day job involves breaking automotive things by fatigue, so I’m very aware how even little flaws can have big significance).
Other reasons:-
No jig.
Not got a beefy enough gas setup at home for heavy cast fork crowns.
Not done much with lugs so would need to do lots of practice parts to cut up and check for brass flow.
Not very clued up on available fork blades (esp what works with discs).
Too many things to get wrong with disc forks (too heavy and stiff vs too flexible, failures, resonances etc). I’d need time and money to experiment.
Saying all that, my next two projects (a 36 er and a modern lightweight recreation of a mate’s vintage bike) will need custom forks so guess I’ve got to start learning 🙂