My experience with steaming vegetables is that a no-name insert, such as the one in the left of the image, inside a nice-brand covered pot steams vegetables very quickly (carrots in 5 min, green beans in 7 min, etc).
A no-name steamer (*) that sits above a boiling pot, such as the one in the right of the image, steams vegetables, still in my experience, in far longer times (carrots in 8 min, green beans in 11 min, etc).
Yet the second kind is much more convenient to operate. It can be lifted with handles, and it's possible to stop the heat underneath promptly.
Is it possible to find an above-pot steamer that's as efficient as an inside-pot steamer, or is it simply that the steam envelops food much better in the first kind of steaming?
This matters a lot when "parboiling" veggies for a subsequent quick stir-fry (to get the best of both worlds: health and flavor). The time for each kind of vegetable needs to be rather exact.
Worst of all is homogeneity. In the first style of steamer all the vegetables (of one kind) remain at exactly the same level of crispness. The second style, for me, cooks food at the center much faster than in the periphery of the pot, which requires attending to the boiling to turn the food.
(*) The manufacturer of my vast set no longer produces a steamer exactly matching the pots in the set.