High-speed signals are incredibly fragile. The chips themselves are a solved problem, but you still need quite a bit of skill and experience to design a PCB for them. It's quite easy to mess up in subtle ways which look like they should work, but actually ruin your signal to the point of being completely unusable.

If you're designing a board, not being able to look at its signals is a major limitation. Is something wrong with the transmitter, receiver, cable, connector, pcb, firmware, driver? Who knows! It doesn't work, and that's all you're going to get. Have fun randomly tweaking stuff in the hope that it is magically going to work.