There was a pro on FB (yeah, I know) that had an visit from IRS, and she didn't have the necessary documentation for the returns IRS chose to check at her office and got nailed pretty good, over $20,000 in penalties, if I recall. Of course, she was diligent, just in her mind, not on paper. /s
IRS says they pick preparers and audit at least 25 returns based upon them preparing [a relatively large number] of returns with a "high chance of error."
https://www.eitc.irs.gov/tax-preparer-toolkit/compliance/auditing/auditing-for-due-diligence-compliance
(Regarding "high chance of error," most DIY returns I've seen fit that bill.)