When I use 2 uppercase letters as custom Axis Tag, the resulting font crashes Adobe software 
Glyphs could add 2 trailing spaces or give a warning, as it does when using lowercase.
Following the OpenType spec, a custom axis tag must start with an uppercase letter followed by uppercase letters and digits; spaces are reserved for registered tags.
Nonetheless, you are right in that Glyphs should handle cases with fewer characters in a tag more gracefully than it does now.
I believe the spec does not explicitly say that spaces are reserved for registered tags, it is not clear. But I can make a custom axis with two trailing spaces and the font works fine in Adobe and Goggles. (just can’t build that in Glyphs)
I have fixed it. Until the update is out, you can set the axis tag by script. Then the validate is not applied.