People are eager to offer what they think you should have done. They will describe in broad strokes how your product would be better if only you had done "X". They won't ask about your goal but they will project where your concepts are leading and how you should move forward. When pressed they will be shy for details but they will sow enough doubt that you'll question your path and it'll slow you down. We wrestle with enough doubts on our own. Beware the vocal minority. Don't give them too much power over your vision.
It's easier to identify what doesn't work than to articulate what is perfectly okay so most people's default mode is critical. Some people are just invested in being the one who can spot faults. They feel like they're accomplishing something by pointing out faults - they feel smart and sometimes they feel like they're helping.
If someone expresses doubt don't take it as the gospel. Be critical of feedback, ask for details and test your ideas against multiple people.