Lots of valid arguments above. A lot of the "problems" (shimmy, etc.) depends on the exact geometry of the bike and the rigidity of the frame and wheels. As a bicycle tourist and utility cycling, I have lots of experience with light and heavy loads on various types of bikes.

If you are carrying heavy loads, especially on a long distance and/or on difficult terrain, splitting the load evenly between front and rear works best. Anything between 40-60 to 60-40 works fine and if you have a rigid bike, sturdy racks and the proper weight distribution, riding the bike loaded with 70-80 kg of stuff will feel as good as riding it unloaded... except for lousy acceleration!
For load on the front wheel, I prefer a lowrider rack because the load is centred on the fork rotational axis; much easier on steering than a top rack with panniers in front of the fork rotational axis!

For lighter loads and especially for shorter distances (less than 4-5 km), I prefer a rear rack and panniers, but mostly for practicality.

– Load doesn't need to be split evenly between right and left panniers. No handling issues when I carry 10-15 km more on one side than on the other. On the front wheel, the load needs to be evenly split between left and right panniers.

– It's possible to attach things outside rear panniers, to have some sticking up, some loosely attached on top of the rack... without any safety hazard.

– It's easier to find a good but cheap rear rack and panniers than a good but cheap front rack and panniers. There are exceptions, though.

– Lowrider panniers get very messy in the snow.