The chain stitch is actually a pretty strong stitch, and ENO uses very heavy thread and triple stitching. I wouldn't worry about it failing. In reality the seam probably doesn't take much wear (my Yukon Double doesn't in that area, and I use it often). Besides, the ENO's been around long enough that if it was going to, I'm sure we'd have heard stories by now.
There are different variations and uses for a chain stitch, and closing sac's off is only one. Just like a basting stitch is a version of a lock stitch, it's all about how the machine is set up to sew, and the intended use of the stitch. I wouldn't be too quick to judge simply because it's a chain stitch.
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