The CRC says frozen potatoes require a hechsher (even though many frozen veggies do not).
The Scroll K says frozen potatoes do not require a hechsher.
Why might frozen potatoes require a hechsher?
A Google image search on frozen potatoes shows only cooked/roasted/prepared potatoes (although this might depend by region). If frozen potatoes are cooked, that would be enough reason to justify a hekhsher. R Elefant, the COO of OU Kosher, said that in some cases, the same steam pipe is used to heat vegetables in one industrial pot, and pork and beans in another, with the hot steam pipe connecting the two ! (listen here at 22'10).
But even if only a part of them are cooked, no responsible hekhsher would give a blanket allowance to potatoes if some are cooked. The Scroll K clearly writes that processed potatoes require reliable certification. I read the ones not requiring as being frozen raw and not containing oil.