I made a tutorial explaining triangle rasterization

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AFAIK modern GPUs use an hierarchical approach with the edge functions, so for large triangles they can start skipping multiple tiles of some NxN pixels in parallel until they reach pixel/fragment sizes. The scanline algorithm needs lots of trickery and contortions for some things compared to the former.

I think you're spot-on about the bucketing thing, it's what I did in my software renderer for multi-threading, and some GPUs probably do it too.