I used the Istanbul -Burgas route and back several times. The buses are clean, there is a snack. The stops are sanitary. There is no queue at the border, but it can take from half an hour to a couple of hours. On the way, they can make unplanned stops to transfer goods, maybe they earn extra money? It is somewhat unnerving, since stops are in the wrong places. As it is, everyone treats passengers kindly. I recommend.