Kopsis

If you look at how the fork applies forces at the bottom end of the steer tube, you'll see that the load is applied as close as possible to the bearing. Keeping the lever arm short means the bending moment is concentrated in the fork legs (which are designed to flex) and not the steer tube (which isn't). At the bar end, you ideally want to have the same thing. Adding spacers increases the length of the lever which increases the bending moment on the steer tube. For aluminum steerers with a limited fatigue life, time to failure will be reduced. For CF steerers, the effect of the increased bending moment is highly dependent on the layup. Steel pretty much doesn't care

Now, how much effect a given amount of spacers will actually have on an alloy steerer is hard to determine. It's not like going from 40mm (the typical recommended limit) to 50mm is going to cut the fork's design life in half (and even if it did maybe you're now down to 10 year MTBF). But 40mm to 125mm is about a 200% increase, so the effect is probably not negligible. I'd guess in that case you probably are taking at least 50% off the design life. That doesn't necessarily mean it's not safe, it just means that you may need to think about replacing the fork sooner than you would otherwise.