The Excel Upload widget definately copes with nested row and column dimensions. The only requirement to get this to work is that in the view definition the grid dimensions (and the stacking order) must match to the Excel file and the content of column(s) A (+n) and row(s) 1 (+n) must contain principal names or valid aliases of the grid dimensions.
The Excel Upload also copes with string as well as numeric data. AFAIK there shouldn’t be issues with mixing string and numeric data in the grid, or at least I haven’t heard of an issue with this before.
What the widget doesn’t cope well with is blank space in the column and row headers. It should be just a single continuous grid. Also somethgin to be aware of is that empty cells in the grid will be skipped and excluded from the batch. (So if you want to clear a numeric cell you must include a “0” value in the Excel cell and likewise to clear a string a " " value.) As the update is in a batch any invalid row or column headers (or a string value to a numeric cell) will fail the entire batch. This is something which isn’t very user friendly and we are aware of and there is an ER already to add additional pre-validation and better user error feedback before the commit (just a matter of prioritization, as you know there are always lots of competing things to work on).
With the available details this is going to be difficult to solve as to what exactly the issue is. It looks like the data preview is apearing and the failure is on the commit? I think it would be good to have a quick 1:1 support call to get more info @wwang are you able to take this up?