It's a bit more than "unscrew this, replace that". Even the relatively simple maintenance procedures often require multiple steps and referring to bunch of charts and tables. Something small like using incorrect torque setting can have disastrous consequences. Most of mx manuals are not available in languages other than English. And it has nothing to do with competence of overseas mechanics, it's just humanely impossible to memorize those procedures. So yeah it's kind of a big deal.

PS: Also from personal experience I can tell that US does have significantly higher training and work standards than certain other countries when it comes to aviation. I immigrated to the US from overseas, am currently employed in aviation industry and hold a dozen of aviation certificates and ratings including those from FAA.