I've never met "some say" before, but have sprayed and killed a lot of entire plants, weeds and small trees with Roundup before. There's been a few times it didn't work, but usually it worked if the leaves were not dusty. It even annihilates blackberry plants roots and all if applied around September to plants that are not badly stressed. About 40 years ago I maintained both pesticide consultant and pesticide applicator licenses here in Oregon, and repeated college classes to maintain my arborist CEUs. So I have a decent foundation about what works or not.
Anyway, Roundup is a TRANSLOCATING herbicide typically applied post emergent, and it will translocate to roots. It's one of few ways to kill small trees that have grown their roots amidst the root ball of desirable shrubs, where digging won't resolve the problem.
But if glyphosate has failed for somebody, that's worth keeping in mind. There's reasons or conditions that may have worked against them at times. Could be dust on the leaves. Maybe they left the site an an unknown downpour occurred while they were off site. Maybe the foliage and petioles were too far into autumn transition