Inexpensive machine built wheels are commonly neither very true or properly tensioned. You save money but have to do the final touchup yourself or the shop you bought them from does it for you.

My experience with prebuilt wheels is with two sets from Colorado Cyclist, one pair from Wheelsmith and one set of Shimano WH-R560 wheels. These all came perfectly true, properly tensioned and needed no attention for thousands and thousands of miles. However, none of these were cheap.

I have worked on a couple of sets of inexpensive prebuilt wheels for others and most of them needed some truing and tension adjustment.