Hey Greg,
just in case you do want to support yarn at all it seems that for yarn 2+ (v4 just came out hence I remembered to upgrade from v1) run-p values need to be wrapped in single quotes, e.g.

"watch": "run-p watch:**",

becomes

"watch": "run-p 'watch:**'",

with no issue for npm.

Background: mysticatea/npm-run-all#200 (comment)