I believe the Valkyrie is actually quite a good unit, but your first point it the biggest hazard in the way.
15 minutes is just too long, especially when you have just two dark barracks. Why cook Valks when you can cook loons or wiz for 8 minutes spread over four regular barracks?
Or why spend 15 minutes cooking a Valk when a Hog takes just 2 minutes?
I really beleive if the train time was halved [ish] that would make the biggest difference of all.

