I've experienced this firsthand, and can corroborate Kenji's recommendation against using flaxseed oil.

I drank the flaxseed Kool-aid (courtesy of this article) when I first started purchasing cast iron ~6 years ago. The flaxseed seasoning is incredibly hard, which is great, until its brittleness causes it to flake off. I was constantly losing bits of seasoning nearly every time I used a pan. (my working theory is that thermal expansion/contraction causes parts of the polymerized coating to lose bond with the steel over time).

I finally got sick of dealing with it, going at all my pans with an angle grinder and flap disk to remove the old seasoning and started over using bog standard vegetable oil. I've not had a flaking problem since. I do need to re-season more frequently, but that is easy and quick to do.