That’s definitely not true, and I’ll give two incredibly common examples:

1: arraylist. A library will probably do better than you. It’s already written and common.

2: webservers. Sure, you can write one yourself with sockets and a bunch of buffers and string management… or, you could use one of the 100s of existing ones. Which one? Probably a stable and established one.

How many times has the stable and established X surprised you with some unexpected behavior?

Once upon a time I wanted a URL /like/this/with%2Fslashes/ where a path parameter could, in rare cases, contain an encoded slash. Legit according to every RFC I checked. Works fine in browsers. But I couldn't get the web server to not convert it to a / before hitting my code. And woe betide thee who wants to make nginx return a custom error code 444.