Mroonga doesn't work in 10.1 tree.
In particular, it creates a wrapped InnoDB table using ha_mroonga::wrapper_create() passing down the TABLE_SHARE from the parent Mroonga table. This kinda worked in 10.0, even though it's not strictly correct — for example, share->db_plugin was Mroonga, not InnoDB. In 10.1 it does not work at all. In 10.1 InnoDB uses engine defined table attributes and it expects share->option_struct to be set to a correctly filled option structure. But that TABLE_SHARE was created for Mroonga, it does not have InnoDB-specific options. InnoDB crashes trying to resolve a NULL pointer.
For a correct solution InnoDB needs to get its own TABLE_SHARE object.
May be there will be other issues with Mroonga in 10.1, I don't know, that was the first one that I got.