Founders give enormous weight to what a mentor says. That's exactly why you need to be precise about what kind of statement you're making. There's a difference between a fact (“this regulation applies to you”), an experience (“when we did this, it took nine months”) and a hypothesis (“I suspect enterprise buyers won't care about this feature”).
All three are useful. The problem is when they blur. An experience presented as a rule can send a company in the wrong direction for months. A hypothesis delivered with the confidence of a fact can shut down an idea that deserved testing.
Labelling your input lets the founder weigh it properly and decide what to verify. It also models the intellectual honesty you want them to bring to their own decisions and to their investors.