I decided to refresh my experiences with extension tubes and extenders on my 5D MK III and 100L. I used a cheap 13mm extender and a Canon 1.4X MK III on my 100L. It would not autofocus at all, it just went into a high speed hunt mode, almost a vibration.
Then, I tried live view with live AF. It slowly focused, sometimes, but was only suitable for use with a tripod.
Just using the extender was much better, no issues focusing, but still difficult when hand held. Obviously, I got the best results with the bare lens. In the end, I just deleted all the shots, I wasn't happy with any of them. I'm impatient, and do not want to setup a tripod or slider. A thunderstorm was brewing in any event, so I gave up.
I was playing around with my old but newly acquired 5D classic this morning and my 24-70L MK II. Its not particularly good for close-ups. a moth flew right in front of the lens as I was trying to focus on a bumble bee, so it got in the photo.