The teammate who complains the most about the plan might be the one losing sleep over it. Ask before you tune them out.

Complaining sounds like disengagement and is often the opposite. The person who has actually thought the plan through is the one who can see where it breaks, and having no good way to say that turns into friction. Tune them out and you lose your earliest warning, plus the person most invested in it working. The tone may be bad. The information underneath might be the best you have.

Next time someone picks at the plan, ask what they think goes wrong first and listen to the whole answer. You may still disagree, but ask before you decide they are being difficult, because the two look identical from outside.