some people have told me that putting in a shift kit will make the trans shift hard and not smooth...is this true?

I believe that's what a shift kit in an automatic transmission is supposed to do.

that seems like that would be bad on the drivetrain and u joints?

Shift quality depends on the amount of throttle given, slow throttle, soft shift; heavy throttle, hard shift.

No it's not hard on the drivetrain.