When using set with the /stats command, the selector doesn't work with anything other than UUIDs or usernames.
Example:

Formatting: /stats entity <selector> set <stat> <selector> <objective>
Command: /stats entity @e[c=1] set SuccessCount @e[c=1] DummyObjective
The first selector works with usernames (blockerlocker) UUIDS (2e7c2061-704d-4993-b452-22347b899246) and selectors (@p, @a, @r, @e, [data tags]) However the second selector only works with usernames and UUIDs, selectors don't work. Likewise, block selectors don't work either.

Formatting: /stats block <x> <y> <z> set <stat> <selector> <objective>
Command: /stats block ~ ~ ~ set SuccessCount @p[type=FallingSand] DummyObjective

There's only one selector in this command, and it doesn't actually work with selectors, again only UUIDs and usernames. Some sort of bug, I dunno, fix it please.