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My Flexispot EC1 (cheapest of the lot with 2 part legs) is absolutely solid. No wobble.
I have an 8kg 43" monitor on it (along with laptop, keyboard, mouse, fan, lamp, clutter).
I guess any wobble will likely be down to the floor surface. If that's uneven then you might get wobble. Mine is on carpet.
You also have to assemble the desks yourself, so it could just be people putting them together badly.
[EDIT] 140cm width desktop on it.
Flexispot users, are your desks wobbly? Saw a review that said the cheap one wobbles if it's got a lot of weight on and I'll have 2 monitors on it.