I don't so much doubt, as much as I become an insatiable skeptic, when I hear "all X are Y," outside of hard sciences and pure math. It's so rare that it actually holds true. It's usually more of a practicality when giving advice or trying to convince someone of something. Often times, there is nothing wrong with that. It is what it is. But sometimes it can lead to dangerous assumptions.
I especially have doubts when the phrase refers to people. I want to know what you really mean, what you are really trying to say, what your intent and your assumptions really are, or what real life context it is said in. Because, when you say "all X is Y" or "all X are Y," all that stuff really matters. For real, for real.
It's a phrase that draws enough personal scrutiny from me to be pretty sure I struck it from my vocabulary long ago. Probably since I got into reasoning, analysis, and writing (which has made up for the bulk of my academia and career). Whenever I hear it in the real world, I assume it's a practicality to get things done or get a viewpoint across far more often than I assume it is a truth or near-truth. I need something near proof or reason for me to think of it as the latter.
For anyone who knows me and ever hears me say it, or has a conversation that shows it, you need to point it out and correct me where needed. You better show me or say it then. I'd be happy to go over as much.
In general though, broad based comments like these can be dangerous when they haven't really happened, or are not explained or considered within proper context (context: another huge thing for me). Those kinds of words need to be handled carefully, because they are pregnant with all sorts of assumptions that lead us to conclusions that have no basis. "All X are Y" is exactly how the financial crises, slavery, and over-extreme partisan politics happened.
These are the kinds of reasons I'm so huge on asking questions and addressing people directly, to clarify assumptions. Personally, professionally, whatever. That sort of action makes assumptions negligible, useless even, makes exceptions clear, clarifies things for all parties. Discussions are forced to become something more reasonable than shouting matches about who is right. To guess that we know for sure what's going on in the next person's head is.... just a guess.
I'm always going to take the opportunity to do better. I actually revel in, enjoy, being proven wrong as a chance to do better. It puts my mind to work. But I'll also open my mouth when things don't seem to make sense. I don't think a lot of people are very different from me.
I think, before you say something like that, it might be best to talk to the one or few people the comment is really reserved for. You might learn something. Those are the instances when I always do.
But don't hide behind "all X are Y" without merit, if you can't really be sure it's true or explain for near certainty why it might be. That's tough hill to climb and a long, long, looooong way to fall, if there's not much truth underneath it.