When someone suggests an idea, there’s usually discussion about it, including critiquing it.
Valid criticisms are things like “there’s a problem because X means Y will happen”. Invalid criticisms are things like “it just seems like a bad idea”. It’s all about specifics.
The rare time when criticism can be vague is when it’s a situation that the criticiser has personally experienced. For example, a parent telling their child “don’t hang out with a certain group of people”. Because of specifically applicable experience, the parent knows that some actions inevitably lead to bad outcomes. It could be getting arrested, it could be a car crash, it could be any number of things, but many of them are bad. Specific experience and trust are the times when a criticism can just be “just trust me on this”, and still be valid.