Is it a myth that some proxies are more stable than others or is that actually true? But then I don’t really get why there’s an option to buy proxies that don’t work well.

No, it’s not a myth. They exist because someone might need a proxy just a few times. Or someone wants to test how a proxy works. Not everyone needs super-expensive proxies, and these kinds are the most stable.

Probably residential or mobile ones. When you use these proxies, you look like a completely normal user, and most site security systems do not recognize that you are trying to bypass a block. There are time tested proxies https://y2kfonts.com/ipv4-proxies/ that are still relevant and work great. Each proxy type fits specific tasks and gives a different level of protection from blocks.