We flew Direct Air out of Worcester, Mass, into Sanford, back in April, and again in Oct. Both times went fine. BUT....since DirectAir flies on so few days, with only one flight on that day, if there is a mechanical or weather issue, you could be stuck. I have one friend who had a delayed flight due to mechanical issues and they brought in another plane but it took a long time. And, there is nothing to say that DirectAir will always do that.
I won't fly with them if there is any chance of bad weather. Way too risky.
As far as boarding without a seat already chosen, I would imagine that they board those who have paid to choose specific seats, and then they allow the open boarding..just not sure how that works since I always pay to pre-select my seat. Although that doesn't always get you the seat you choose.....flying home in Oct, we found that our seats had been given to others...not sure why. It wasn't a full flight, and we got to the airport over 2 hrs prior to our flight. I wasn't happy. We were given exit row seats though....and because there were 4 of us, and each row had plenty of room, we were able to sit 2 in each row, with the middle seat vacant. I did complain about having paid for specific seats only to have to sit elsewhere...I was refunded the money we spent to choose seats.
I think Direct Air is fine...as long as you realize how they work. They usually run out of small airports, which may not have the usual amenities, especially for early morning flights. And you have to pay addtl if you are flying into Sanford...heading to WDW....either pay for a car service or rent a car. Many have booked Direct Air for the terrific fares and then found that they had to
budget car money as well as for the addtl time to get to the more distant Sanford Intl. Airport.