But in Lightroom, stacking is context sensitive. You can stack images in a folder, but that doesn’t stack them in the collections they appear in. You can stack images in one collection, but they won’t be stacked in another. And smart collections don’t support stacking at all.
Lightroom’s ‘localised’ stacking is a real headache – it’s often very important to know if an image is stacked with another, but where Aperture will always show you, Lightroom will often give you no clue.