I think the emphasis so far has been for python devs:
"Don't think of BUILD files as python. It's a DSL for configuring pants metadata" - the emphasis there is that imports won't work, so a lot of pythonic ways of doing things won't work.
But, with StackStorm, I use the fact that BUILD is python as a selling point, because we're a python project, so that makes it feel less daunting to learn.