The only downside is if you want to switch between sitting and standing, but frankly it's easier to just use a laptop for sitting for a little while until you get used to standing for longer.
And as far as a criticism of it being "wobbly" -- i've seen automobiles perched on top of plastic milk-bottle crates. Not that I'd recommend it, but surely one will safely hold a relatively breezy laptop and a monitor (15kg total?)
Amazon even sells a multi-level cardboard standing desk for $25 that's perhaps a touch more attractive than a milk crate: https://www.amazon.com/Spark-Ergodriven-Perfect-Standing-Med...
So give it a try. See if a more neutral head-up posture and straight spine are agreeable. If so, consider an upgrade to a "real" standing desk. If not, no real risk or loss.
It's slightly wobbly at max height, but otherwise not bad.