I doubt that many equipment manufactures would be happy about honoring a claim involving DIY or boutique fuses.
Agree. When I posted in this thread, the real problem with boutique fuses is not the voltage rating, but the speed of response. Many boutique fuses are a little sketchy on fast/slow blow rating. And even if the speed is mentioned, the fuses are not really seriously tested to be 'standard speeds.
So in a circuit needing fast and you go too slow.. you equipment becomes toast.
And agree the manufacturer is not going to be happy trying to claim a warranty due to an improper fuse being used.
So IF you have a real clear promise of rating and speed etc.. Well, it still is your burden if it all goes wrong.
And I agree the best standard fuses used to be those 'lightning bolt' appearance fuse element. They were in my Forte 4a as stock.
Those allowed several just very small fuse rated bits in the longer fuse element. Rather than a full one inch wire being the fuse rated element.