Once you have a flat surface resting on the log supports, moving the log or cant in and out is pretty easy. It's not unusual to have to do this to allow the saw sled to clear a "surprise" knot or curve along the guide rail. Cutting the flats on an irregular log requires some additional wedging to keep it oriented in the right direction, but there is a spiked bumper that can be driven into the far end of the log that helps a lot. There's a big advantage to having the log supported at only two points, since the shape between the supports doesn't matter as long as it allows clearance at the guide rail for the sled.
There are limitations on the size of the log - 1100 lb limit & the log can't be so curved that it won't sit on the log supports without touching the guide rail. I certainly wouldn't try it with that log you show, but I have milled a 7 ft, 16 inch diameter oak log with a pretty good curve to it the way I described using a 24" bar. The big tradeoff for this type of mill is the effort of getting the log up on the mill vs. the more comfortable milling height and easy adjustability for multiple cuts. I have not found curved logs to be any more difficult to move up on the mill, but I often cut larger logs in half or quarters on the ground before loading them on the mill.