Here's some insights for PRs created within the last 120 days for the mentioned repos.
88 PRs created: https://reviews.gitsense.com/insights/github?p=days-open&q=a...
70 merged: https://reviews.gitsense.com/insights/github?p=days-open&q=a...
7 rejected: (closed/not merged) https://reviews.gitsense.com/insights/github?p=days-open&q=a...
Interesting take away is, the merge rate is quite high, but since we can assume these PRs were created for a reason, can we also say 7 not being merged is also bad?
Full Disclosure: This is my tool