The lighter cylinder will prove a plus in regards to the durability of the cylinder stop and reduced notch peening, lower mass means less stress in these areas.
As for the thinner web between chambers, best I can say is that I haven't seen any issues posted concerning this aspect. IMO it's probably safe to assume that the cylinder is plenty strong enough because enough times has passed that if this were an issue we would have heard about it.
In timing, IMO that extra chamber is acually a positive. Because the cylinder doesn't have to rotate as far this means that the mechanism droving the cylinder has a bit of extra motion available. I have a model 620 which is also a 7 shooter and of all my revolvers it's the only one that times out perfectly on every chamber with a LEFT HANDED slow motion trigger stroke. In addition it clicks in a bit "early" with either left or right hand. Basically, there is more "cushion" in the timing to allow for wear. Contrast it with my 625 which times out within a gnats eyelash of being late with the right hand and that "cushion" seems quite large.
Finally there is the extra bolt cut. That is also a distinct positive because due to the odd number of chambers the bolt cuts are BETWEEN the chambers instead of the chamber wall. Much more steel under the cut makes for a stronger chamber.
Sum it all up and IMO the 7 shooters will actually last longer between service intervals than the 6 shooters. However, one defect in my case seems to be a distinct lacking in the ability to count to 7, nearly every time I take my 620 out for a range session I find myself dropping one unfired cartridge in my brass box for each cylinder load.
