It's not about backend or frontend, it's mostly about languages. If you're using niche stuff or languages that don't have an online or digital presence, the LLMs will be confused. Stuff like assembler or low-level C aren't really in their vocabulary.

C#, Go, Python for example work perfectly well and you can kinda test the LLMs preference by asking them to write a program to solve a problem, but don't specify the language.