I would expect that anything that requires modification or reissue of the ticket won't work. Complementary seat selection should usually be fine (but, again, doesn't work on JL, so not always). But if they have to collect a fee, that fee is going to get attached to the ticket (I'm not sure if it goes in the fees section of the existing ticket or if they create another document with the fees collected and somehow link them) but it doesn't surprise me that many airlines can't handle this if it's not a ticket marketed by them on their own ticket stock.

I'm sure it works on some airlines but it doesn't work on a lot of them. In general it's just asking for problems.