It can certainly save a lot of stress and worry for family at a difficult time if everything is organised and pre-paid. And can be financially beneficial if you have a need to reduce your savings.
However, I had a bad experience. A relative had pre-arranged and pre-paid his funeral. Undertaker, cars, church, burial, reception afterwards - all organised.
Jimmy had been a joiner - the type of guy who would admire some craftmanship "That's a nice bit of wood, that is..."
He had specified a solid wood coffin; quite an expensive one, the same as his wife had been buried in.
The night before the funeral, his body was taken to the church to lie overnight. When we arrived, the first thing I noticed was that Jimmy was in a formica coffin!
Mentioned it to my wife, who couldn't believe it, but we didn't want to say anything in case we upset Jimmy's immediate family.
Pub afterwards, mentioned it to brother-in law - equally appalled, but didn't want to say anything in case it caused upset.
A few drinks later, we mentioned it to immediate family - yes, they were appalled too, but didn't want to cause a fuss.
A phone call was made, and next morning when we arrived at the church, Jimmy was in a new box.