I've carried significant loads (but didn't weigh them so can't quantify "significant") in an Acorn Large front bag sitting on a small Nitto rack on a 650b bike equipped with 42mm Hetre tires. The trail is 30mm. It rode well. Turning took more effort than it does with lighter loads up front, but the bike held its line well even in the wind.

I've also carried lighter loads in an Acorn Boxy Rando bag on a Velo-Orange front rack on a 700c bike with 28mm tires. The trail is 56mm. It also rode very well. Turning wasn't negatively affected---at least not to the extent I could tell---and the bike held its line well even in windier conditions.

Finally, I've carried light loads in an Acorn mini-rando bag (no longer made; it's like a small rack trunk for the front) on a small Nitto front rack on a 700c Sam Hillborne with 33.3mm Jack Brown tires. I believe the trail is in the high 60mm's. I didn't like it much; even in fairly mild crosswinds the bike was wobbly. I had trouble holding the bike to a straight line even with the small winds.

That's all I can add to this discussion, but based on my experience I've concluded that a load on top of a front rack is okay on a low- or mid-trail bike, but destabilizes the steering on a high-trail bike. Others have claimed no negative effect of higher front loads on steering on Sam Hillbornes, so my experience differs from that of most people I've seen write about it. I've not tried front panniers on any bike yet, so can't comment on that beyond what I've read from others.