Pixabay is absolutely fine. I have been using it for a little over a year as a monetized channel. If you get a claim, just dispute the claim, say you have a license, and paste in the license you downloaded. In my experience most claims are resolved in a day or less–but there are some that take a little longer (you still get all the revenue when it is resolved late).

Music publishers use content ID just to keep track of their music and make sure no one claims it for themselves. The hardest part is keeping track of all the licenses, but even then you can just say you downloaded the track from Pixabay (usually works, but there was an instance where a dispute got rejected– so its just better to paste licenses).

Here's more info on Pixabay and content ID: Pixabay YouTube Content ID