Its horses for courses - depends on the effect you want to produce, the colours you want to use and the type of t-shirts you use.
As I understand it, large flex designs can feel uncomfortable when worn as it alters the breathability of the shirt.
There are limits on how small a flex design can go. Each element must be at least 1.5 mm thick and there must be at least 1.0 mm space between elements.
Have a look
here. This page is from the European site and has not yet been updated with info on Digital Direct.
Not all products can be printed on using Digital Direct. In fact there is a quite limited list for the European site as they are still getting up to speed with this service.
Some of the partners over at the European Spreadshirt have been very enthusiastic about the results of Digital Direct.
HTH