Webbing almost never fails. Everything attached to it does, and all three failure points are visible before you order if you know what to look at.
The D-ring
A welded ring is a closed loop. A cheaper ring is a bent strip with the two ends butted together, and under a hard sideways pull those ends separate and the lead comes off.
You can see the join. If a supplier cannot tell you whether their rings are welded, assume they are not.
The buckle
Side-release buckles are made in sizes and rated accordingly. The failure is rarely a bad buckle; it is a small buckle fitted to a collar sold for a big dog.
Ask what is fitted and what it is rated for. For very large or very strong dogs, or anywhere the collar takes tie-out loads, use aluminium or a metal-tongue buckle instead.
One point in favour of plastic: it releases under load, which is what you want if a collar snags on a fence.
The stitching
Load concentrates where the D-ring and the slider are attached. That is what a bar-tack is for — a dense block of stitching designed to take it.
A straight run of stitching in the same place looks similar and is not the same thing. Look for the tight rectangular block.
Checking a collar you already own
Look at the ring for a join. Pull the buckle apart and look for stress whitening around the prongs. Check the bar-tacks for lifted threads.
Do it at the start of every season rather than after something goes wrong. A collar is the cheapest item in the kit and the only one holding on to the dog.