I'm curious to know how you know that you suck at it?

There are so many different formal rules, informal rules, dialects, accents , slangs and cultures and subcultures and so on across the world both among native and non-native speakers that I don't believe anyone can communicate flawlessly or even effectively in all of them.

I think the only way to assess competency would be to estimate the rate of misunderstanding in communication. Probably weighted in proportion to importance for survival. I'd guess that this is likely to be the case for most languages as they spread outside of any close-knit culture.

I think being widely used on the Internet makes any language suck; face-to-face is one of the best ways for speakers to understand the effectiveness of communication- and to learn and adapt to what works for that audience. Without that feedback loop, communication and improvement is probably much harder.

Human language just isn't like computer languages as there is no underlying instruction set, no compiler, and no objective standard or measure of what does or doesn't work.

TLDR; I think everyone sucks at English, but lots of people 'get by' which is 'good enough'.

What's your language?