Just reporting that I have used SanDisk SDCFJ-1024 CBE 1 Gb cards with no issues. Have used in both J4300 and J2320 type routers.
I'm going to out on a limb here and guess that Juniper specs certain cards so that it gives them a buffer against some knucklehead who goes to 1800cfcards.com or something like that and then opens a ticket with JTAC when said cheap/crap card doesn't work correctly.
From a support perspective I understand this and I get it.
I really cannot see why compact flash card X versus compact flash card Y would really matter so long as the manufacturer is a solid entity.
I can also see where the cost of officially sanctioned CF cards, SDRAM, etc would be a safety blanket for people...particularly those coming from a Cisco background where you might not have the freedom to freestyle your memory if you know what I'm saying.