Considering it's a one-hour drive from Orlando to Port Canaveral, you'd almost have to have some sort of emergency (medical or collision) to miss the boat. (If you are flying in day-of-cruise, you've introduced a whole new set of obstacles to your cruise than the bus missing the boat.) Pre-COVID, it was cheaper to rent a vehicle for a week and pay to park it at the port for a four day cruise than to use DCL transfers for a family of 4. Now rental car rates are up 100%-150%, so that price difference doesn't exist. Still, I've found that you can get private transfers for a group of four or more significantly less than DCL.
Another point is car safety seats, and this can swing either way depending on your perspective. Kids aren't legally required to use safety seats on a bus, so that's a potential cost/hassle avoidance if you are comfortable with that. If you decide that's not best for your kids, then you've got to schlep your car seats onto the boat and off. Private rentals can provide seats, as can rental car companies. Airlines will also check your car seats at no charge, so you can use them in your rental car the whole trip. (And if you pay to park the rental at the port, you can just leave it there and not worry about it until you return the rental to the airport.)
But to me the biggest disadvantage of DCL transfers is the schedule. Departure day is completely taken over by DCL, as you'll be heading to the port first thing in the morning. If you want to do something else that morning (swim, nice breakfast, maybe even rope drop either a Disney or Universal park), you'll be leaving for the port ong before you can do anything else. And then if you are planning anything else after the cruise, you are waiting for the bus to get you to the destination. Now, if your vacation is 100% cruise, this isn't a big deal...but if you are doing ANYTHING else before or afterwards, you might not want to let DCL set your schedule. We're thinking of visiting the Kennedy Space Center after the cruise, because we'll be able to store our bags in the rental car.