It’s not ok to use private balls under any conditions. There are weather conditions where it’s not prudent to use any ball. That’s where the part called being the Captain comes into play. You have to evaluate those conditions. There are obvious things to avoid like high forecast winds and or swell. Tolerance for those should be situation dependent. A Lee shore lowers my tolerance considerably. Even when you act prudently things happen. I have had two mooring balls release me over the years. It can and does happen. If however you pick up a private mooring ball intended for smaller boats in a large north swell in a unprotected area just off a Lee shore expect a bad outcome.