I just spent a couple of hours trying to rig up a set of cables to connect my sound mixing board to a 4-way RJ-45 switch box, so I could switch my nice Heil microphone among two (and later 4) amateur radios, most of which use RJ-14 jacks for their microphone connections.
I’ve used RJ plugs many times in the past, and been somewhat tolerant of them, but this experience changed my mind completely and enabled me to see all of the faults this connector design has.
Aside from the obvious problems(the ease with which the locking tab breaks off, the difficulty of activating it when some types of anti-snag protective boots are installed, etc.), it has several that are even more problematic. The plug needs to be seated firmly in the jack to make good contact, yet the springs that make the contact tend to push the plug out of the jack (and will often succeed if the locking tab is broken); the most fragile part of the connection is the springs, yet they are on the most expensive and difficult to replace side of the connection; and in many cases there is expected to be tension on the cable, which pulls the plug away from the spring contacts, and this leads to intermittent contacts, which is not good for microphones, telephones, or network connections.
Aside from this, I was most frustrated with the mechanics of the assembly. I was using the correct wire (solid 4-conductor phone cable with a round sheath), the correct jack for my use (a 6P6C plug for round, solid wire), and the proper cutting and stripping tool. Yet I still found it difficult to get all the wires cut to the proper length and positioned properly in the connector. And once I was mostly done, I realized that I needed to connect multiple wires together in order for the right signals to appear at both ends of the cable, yet there is no room to do this inside an RJ connector. Given all this, this is clearly not a connector that should appear on amateur radio equipment, where operators are expected to be customizing it and connecting it to all sorts of other equipment.
So, RJ connectors might have been an improvement over the bulky 4-pin telephone jacks they replaced, but they are quite clearly inappropriate for most of the equipment they are now used on today, and I would love to meet the engineer that will design a replacement that can be used on the next generation of telephones and Ethernet networks.