If you are gold or higher, you have star alliance gold which applies equally on NH. The only meaningful benefit you might lose moving from UA to NH is economy plus seating. Since the NH aircraft offer more space in regular economy, that is less of a loss than it might otherwise be. In fact, the 789 they usually fly NRT-IAH has 34" pitch in Y, which is essentially the same as UA's E+.

NH overall is a much nicer airline to fly than UA.
They're not. As jsloan explained, it has solely to do with available inventory.