It is not that they do not work at all, but that the performance is greatly diminished.
Take your garage gasoline can that has one of those yellow vent covers on it.
Try to fill up your gas tank on your lawn mower with the vent closed off. It will only flow out in a glug-glug-glug manner.
Now, take the vent cover off. Swoosh!!! The gasoline flows feely and readily without starving the internal portion of the gas can for air dispalcement.
I would not be crawling inside of your attic through all of the insulation to the narrowest point, just to see if your soffit vents are clogged, unless i already either had a signed contract with you or you paid me for my complete scope of work and cost analysis.
On a cape cod, I can pretty much guarantee that the air flowage is stifled and will need modifications.
On an open attic area, I can at best, only count how many actual soffit vents you currently have, determine the NFVA, then compromise that number by about 50% less to account for dust and other debris inhibiting the fresh air intake ventilation from entering the attic area.
That is the best I could do, because for free, I do not have the time to only do estimate measurements at night time when most home owners are at home. I use those evening hours to set appointments with them, to review the details and specifications.
At that time, if the soffit ventilation had been even slightly in question, I would inquire if they wanted it checked out further during the course of our contracted roofing work. The price for any remediation is the same, whether or not I knew in advance, or discovered it when we had the roof opened up.