"Any concerns over the zip tie popping loose, especially when cinching down the strap?"
I always have concerns about things popping loose and the zip tie is no exception. Here are my thoughts, in no particular order.
(1)When making stuff I always try to attach crucial gear in at least 2 ways (two straps, two buckles, etc.). If one connection fails the other should keep things attached until I notice and fix it. I wince whenever I see someone on the trail with their sleeping bag or tent attached with only one strap.
(2)About a week ago I used zip ties to attach two 5/8" ladder lock buckles to my pack's waist belt. This should give the attachment technique a pretty good test. So far, so good with 30 lbs of weight in the backpack.
(3) There isn't a lot of direct load on the zip ties. They mostly just keep the webbing from slipping through the slots in the buckle.
(4) I've used the zip ties in this manner at least a dozen times over the last year or so and have no failures but one never knows when a faulty one might come along.
(5) I only use the black zip ties that say they are uv resistant.
(6) I was reluctant to cut off the zip tie tails at first, thinking that leaving the tail on might give the ratchet mechanism another chance to catch hold if it slipped a cog. I doubt that it would, however, so I now cut them. Cutting the tails is very satisfying because I'm saving something like 1/500ths of an ounce every time I do it.
(7) One could install two zip ties and pretty much eliminate the chance of failure. Caution-Do this 10 or 20 times and you've added a gram to your pack.
(8) One could replace the zip ties or supplement the zip ties with cord. I tried this for a while but my knots kept coming loose. This might be due to my poor knot making skills, however.