In my opinion, Spider should not use GROUP BY handler unless it is necessary, for performance, and safe. That is because the GROUP BY handler reassembles the query from a general (possibly complex) execution plan, it tends to be more prone to mistakes than execution through the normal handler interface (which should result in a simpler query rassemblement). This is supported by the fact that many of the recently reported bugs are not reproducible on 10.2. I know that there are some problems with access via the handler interface (e.g., MDEV-26111), but I think it could be fixed.