I've just returned from touring on an early 90s Galaxy and agree that it flexes noticeably when loaded. Its enough of a problem for me that I'm looking for a stiffer frame and one with a less lively head angle as climbing at slow speeds can involve a bit of wandering. I'm comparing with my previous tourer, a Stevens (a bit too small for me) and my companions Surley Long Haul Trucker. The chainstay length could also have benefited from being a bit longer, I've average feet size for my height, size 10 and 6 foot, and the frame is 59cm, but I need to place the panniers far back to avoid heel strike. Finally, shimmy - I've had unsettling speed wobbles coming down fast descents which didn't happen on my Stevens.
I know its considered a classic tourer, but from my experience, I'd peg the early 90s 531 ST Galaxys as light tourers. If you come across a cheap bike or frame, it does the job and is a pleasure to ride but I'd recommend not spending lots of money on one of these.