I think it's complicated because people track down the lightest, most high-performance options to put their ass in a sling in the sky, find something that works well but needs to be used in a very specific way, and then make something like a youtube video about how awesome it is (without the disclaimers).
It's also complicated because the worst-case consequence is you fall and die. So even say a .1% chance of something bad happening isn't acceptable. So you need to protect against even unlikely failures. Maybe you could use what you made for years or decades and not encounter a problem - or maybe not.
This is why the recommendation is so strong to use a RCH or other safety harness in conjunction with any DIY saddle. There are a lot of sneaky ways to be dangerous (even if the risk is say 1 in a thousand) - and having a certified, professionally-designed safety harness holding you up is a good way to protect yourself.
If you wanna tie knots, the most foolproof and easiest to use is going to be good old-fashioned static climbing rope. Amsteel is AMAZING stuff, but it's super slippery and knots can slip undone. Webbing with water knots work well too, but water knots will come undone over time, so you need to leave a long tag and inspect before use.
Or keep it simple, always use a professional safety harness, and design to a "seems like it should hold OK" standard. This can be OK as long as you've planned ahead so that when things break you're protected by something that won't.