[_dcwr]

You will also sound very off and instantly recognizable as a foreigner when speaking some of them.

I.e. here's Stephen D. Mull[0] speaking Polish: https://www.youtube.com/watch?v=_nRVUZpi660

He is REALLY good at it for a foreigner. It's 100% understandable (both content as a whole and every single word on its own) but it still sounds clearly off (a bit too soft and very American, Polish accents vary a lot in places but no native Polish speaker has one like his) and he makes some mistakes with conjugations and weird phrasing that no Pole would use.

[0] - American ambassador to Poland at the time who also says he has no Polish ancestry so I assume he learned via this American program at the minimum (plus his own practice in Poland).