It's not illegal - but it's also not helpful, or constructive, and nor is it desirable if you're trying to build a helpful community.
There are other ways of getting your point across, without resorting to insults such as "piece of s*it".
In Chinese, we might call this 家教 - or "upbringing". If you have no decorum, or class - it suggests that your upbringing was poor, or that your parents failed to raise you well.
We often forget that there's another person at the other end of the phone, or at the other end of our comment. In this case, a real-life person who put a lot of work into a product. You don't have to love it - but if you are going to criticise it, at least don't be a jerk, and make some good-faith efforts to work with them through the issue.
It's not exactly like the developer didn't try to help.