Affinity's constraints feature is great, but limited. I just stumbled upon something that would make our lives so much better:
http://westonthayer.com/writing/2016/07/27/layout-experiments-in-adobe-illustrator/
It's even open source: https://github.com/WestonThayer/Bloks
Please Affinity MAKE IT HAPPEN!
EDIT: I just played around a bit and (although working with Illustrator again is a real pain when you are used to Affinity Designer) the principle of "Bloks" is pretty awesome. Unpolished and incomplete but awesome.