Would be nice if the loader / unloader alternate the belt side, specially for consuming itens when stacking up. If you look close, when you have a slow output, you will see that the loader only use one side of the belt, like a inserter does.

Unfortunately the loader mechanics are part of the base game, so the behavior is pretty much impossible to change - part of the performance tradeoff of letting the base game mechanics instead of scripting handle the heavy lifting, so the loaders will unfortunately always prefer to put items on the left side of a belt.

For beltboxes, there's a 'batch stacking' option in settings that you might be interested in - it causes stacking to happen in batches of 4, which means the belt will be loaded on both sides evenly from a beltbox. https://thumbs.gfycat.com/MeaslySplendidBlackbear-small.gif

Oh, thanks for pointing that out, appreciated