No license is required in Pennsylvania, that's for sure. I'm unware of any state requiring a license to abstract. Are you? I'd like to know, so please send a comment our way.
The title insurer is responsible to the consumer for the work of the abstractor. Title insurers can and should use care when selecting an abstractor because the abstract provides the raw data used in examination. It's key to a safe transaction.
Besides having E & O coverage, I prefer an abstractor with 10 or more years experience and someone who "gets it", meaning understands quality and reliability.
I could use cheaper abstractors if I didn't care about quality, but crappy standards are an anathema to me.