H1B workers from the very large and hostile country
(1) Visible projects are frequently reserved for them. If they cannot complete because there is no one to copy from, you will be required to complete for them.
Eventually they will get rewarded for the work you do, leaving you behind.
(2) They are very focused in extracting what you know and bringing "the documents" home.
(3) Once they have a sizeable group, you will probably not survive in the same department, unless you have unlimited knowledge to transfer to them.
(4) They are collecting information that cannot possibly be very useful to them as an individual.
(5) As soon as they gain the upper position, they will legitimately assume they have superior knowledge and nitpick many things you do. Even it is you who helped them on the knowledge at the first place.
(6) They will never make mistakes, because mistakes are always yours.
Have I left anything out?