The problem with not-"not all men" is that it lets men off the hook. They can keep mansplaining, gaslighting, sexually harrassing, and all the rest.

It also is pointlessly offensive, because men get upset when they're accused of something they didn't do.

But wait, didn't we just say they did do it?

Omit unnecessary words.

To hold responsible those who have committed an ill, one must identify the guilty party. This may take an extra word or phrase.

The problem with not-"not all men" is that it frequently lets men who engage in poor behavior off the hook. It also can be pointlessly offensive because those men who did not engage in that behavior get upset when they're accused of something they didn't do.

Do not omit words. Omit unnecessary words.

When someone uses "not all men" to derail a well-targeted discussion about misbehavior, it is that person who has misbehaved: we should object to "not all men" because its use helps the guilty escape.

However, when someone uses "not all men" to reject group punishment or group disparagement in favor of identifying the guilty party, it is those who object who have misbehaved: the objectors are the ones helping the guilty escape.

I don't understand why you argue so fervently to let the guilty off the hook in the second case.

Target problematic behavior.