Is a long, high angle stem safe to use with a carbon steerer? I purchased a hardly used road bike someone else had built up for a deal a few weeks ago, and am a bit disappointed with the fit - it's fine for rides less than a couple hours, but too aggressive for anything longer, for my tastes. In particular, the problem is that my saddle is about an inch and a half above my handlebars, even after inverting the 6 degree, 75mm stem that is installed currently.

So I purchased a 40 degree, 120mm stem to try and get the handlebars as high up as possible. However, I'm now a bit worried that having the handlebars so far from the steerer tube, which is carbon, will offer too much leverage and may cause the steerer to catastrophically fail when, for example, I ride out of the saddle.

Does this seem safe? Are there any guidelines for how long and steep a stem can be for safe use with a carbon steerer?

For what it's worth, I'm about 145 lbs and the fork is an Enve Road Fork 2.0.
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. :)