We still use a 3PJS when my son wants to bring a friend along. It's a great radio and has many features that the 3PK has. It only transmits in 1024 bit PCM or FM so it is not immune to interference like 2.4g systems. You can use a 2.4g module in it but it will be the lower framerate, not 2048 bit HRS. In the 3pk for example you can switch between standard 1024 TX PPM or PCM or the faster HRS TX mode. The FASST module will work in either mode, it's only analog servos that get damaged in the HRS mode.
You can still use digital servos no problem, they just won't have as many steps of resolution that digital servos are capable of.