10/100 HDX is now supported on all models of both EX and QFX, using later code. The situation applied previously to EX4300/EX3400/EX2300 but was resolved with 14.1X53-D40 and all future releases and SW trains. For these products I would recommend only using 14.1X53 or 17.x, nothing in between.
There was actually a "standard" limitation from the vendor chipset, and Juniper need to work with Broadcom SW Developer's Kit to work-around this to get this to work. Of course there were also issues just working with SDK.