Not sure about other people's experiences, but to me the thumb isn't that useful as a typing digit because it can't arch. Multi-row thumb clusters are useless to me because I can't press the upper rows with my thumb without pressing the lower row as well.

To me, a "thumb cluster" should be just a single row of 1.5u keys. Any secondary rows above those aren't reachable by thumb, so they rarely get used, and therefore might as well be removed.

Maybe others have a different experience, but I still use my thumb mainly to press the space bar and enter keys. All other thumb keys are modifiers and layer switches.