For our Mac and Windows monthly patching (just workstations and roughly 400 total), I create my own baselines and action the baselines to my custom computer groups. I generally only leave the action open for a few days. But it seems that 5-10% of machines never check in during the window, probably because they are just off for a few days. Is there any harm in keeping an action open for two weeks to make sure it collects everything it can?
No.
If your baselines were overly long, you could run into client performance issues, but keeping them open for a few days is not a problem.
I don’t recommend “open ended” actions for baselines, but for my workstations I routinely leave the patch actions open for a month. We have +65k endpoints of all different variations of OS.