I have one – used it for months on the road bike for my commute and for a 3 day off-road tour on a full sus MTB.
On both the rack was high (MTB due to wheel clearance when the shock compressed – on the road bike due to the frame size) and with any weight in it felt really off balance when out of the saddle or in a tight turn. Fine seated in a straight line though 😆
Mine never moved one bit – it was a £10 ebay job that has four bolts rather than a QR. I still use it but on a SS hardtail so it sits less than an inch off the tyre and doesnt affect the handling much.
I would second looking at the Carradice or even a bar bag unless the rack can sit just above the wheel.