Geoff,
I have a couple with the Over the Top designs. In particular the older 2012 Murmur (nearly identical to the Kumo, except no top pouch and straight front pouch. The newer murmurs have a roll top design.)
I have used it on about 1000 miles through some rather rough terrain in the ADKs. Peak bagging, a couple 140mi trips on the NPT, through the 5 Ponds area, etc.
Generally I like the design. When the pack is very full, it closes over and compresses the pack. However, with too much pressure, it tends to bulge the pack into your back. With a firm pressure, it produces some curve matching the curve of your back. I also use a 5 layer nightLight pad in the pad keeper to supply stiffness and for sleeping on shorter 2-3night trips. This provides plenty of resistance for the compression of the OTT design.
The design also allows me to keep my sweater, rain jacket under it. I get a lot of intermittent showers, thunderstorms and it makes putting them on and off easy. You can put a bear can under it, but, it needs a firm cinch (see above,) and, it takes up as much room as dropping it in the bottom of the pack. The overall geometry means I do not put a bear ball up there, simply putting it under my compression sack inside the pack.
It seems rugged enough, but I am sure a MYOG pack will be better built. Over the miles I put on it, I did some bushwhacking, and, many miles of trail maintenance. A couple times I need to carry a chainsaw, gas/oil for that. On a bushwhack it can snag if you do not keep it fairly tight, though. The upper corners (once cinched down) can be annoying traveling UL because I run out of cinch adjustment to tighten it down sufficiently, the pack itself is too big. It is not the best for the combination UL travel and bushwhack. A smoother roll top, dry bag style is smoother.
It does keep rain from getting in the pack. Better than the typical drawstring type closure, provided you waterproof the material. But on a canoe trip, I dumped it in a bad landing and it did let a lot of water into the pack. It leaves the main body open under the lid. Good in rain, lousy if it gets submerged. For hiking it is excelent, for boats not too great.
The Murmur was my main go-to pack for 5 years. The lower cord locks pulled loose twice. I finally just sewed them in. I used it more before I got the HMG Windrider 2400. With no insurmountable geometry problems, it is nearly water tight in any submergence. But, it has the same problem iff you use the lower lid mounts. I just roll it down fairly tight and clip it together dry bag style to avoid the arcing/bulging that can occur. I cut the lower buckles/web strap off and sealed the seam holes because they were just added weight.