I would not call it ethical/unethical but professional/unprofessional instead.

You second note is the one I would choose because includes no opinion and provides the relevant information to the next developer.

The first note sounds to me as a rant more than a useful comment...

As a general rule, I try to provide at least some pointer when writing code that has any risk or is a hack to workaround any external issue; this is something I found useful for myself and is more so for someone that did not develop that (ugly) code.

I would not call it ethical/unethical but professional/unprofessional instead.

Your second note is the one I would choose because includes no opinion and provides the relevant information to the next developer.

The first note sounds to me as a rant more than a useful comment.

As a general rule, I try to provide at least some pointer when writing code that has any risk or is a hack to workaround any external issue; however, this is something I have found useful for myself and is more so for someone that did not develop that (ugly) code.