basically, ask yourself: is your response friendly and helpful and likely to motivate the developer to consider your issue, or are you venting your frustration. if it's the latter, then please just don't.
see here for example:
https://news.ycombinator.com/item?id=31945618
i made a similar comment a few weeks ago:
https://news.ycombinator.com/item?id=31679373
users should be free to be critical of it
absolutely not. you can say: "unfortunately, i have this problem, so i can't use this, i am going to go use something else". but that's about it.
criticism is the bane of our society. criticism hurts peoples feelings. criticism causes burnout. FOSS maintainers are volunteers and absolutely do not deserve that kind of criticism for just developing software. (that doesn't put them above any criticism. if they are rude or mean to their users, then that may well be criticized. but not if they ignore a problem or are unresponsive. they may well have other things in their life that are more important than to care about your software issue) (and if the developer is paid, direct any criticism at the company, but don't beat up the employee who may not have the freedom to choose what issues to work on)
Being open source doesn't mean nobody should dare criticise you
if you didn't pay for it, it means exactly that.