Is a long, high angle stem safe to use with a carbon steerer?
Within "normal" standards of long and high angle, yes it is safe. 

While there are some stems which use clamping styles which are not carbon safe, or other restrictions, these are usually clearly stated.

No restrictions are based on length or angle of the stem, that I am aware of.  I have been a certified professional bicycle mechanic for 20 years, so I would think I'd have seen it by now, if it were an issue. :)