How to write div:not(#foobar) > .form-type-checkbox > input[type='checkbox'] + label with sasson

Well, it is very much possible that very complex selectors will make the php compiler go bananas.
I wonder if you could break it down and see where exactly it fails, that will help a lot.

Have you tried using the ruby compiler? You can go CLI and use the command line to sass-watch http://sass-lang.com/tutorial.html or be all fancy and get an app like http://fireapp.handlino.com/

I must add that if that is how your selector looks, you better off writing a theme function and add a class to this label, don't you think ?

Marking as a Phamlp compiler issue.
The PHP compiler, while serving us well, will always lag behind the original ruby-compass compiler, this is why the upcoming release (v3.x) of Sasson will use assetic to compile Sass using the original ruby compiler.
Unfortunately, bugs with the php compiler is probably not something we will have time to work on anytime soon, since most of the effort is going into the v3.x branch which uses the original ruby-compass, so marking all Phamlp issue as postponed.
If #1825006: Phamlp can't get arguments as @media queries values is solved, we might find a way to support the php compiler as a fallback for people developing on a server that can't install ruby (we recommend developing locally, on a local server with ruby and compass installed).
Any help with this will be most appreciated.

The PHP compiler used in Sasson 2.x is no longer needed in v3.x and I don't see me or anyone else getting to maintain it.
Marking all relevant issues as won't fix. sorry.
You are welcome to try the much more powerful v3.x