analag said:
Extremely vast question.
FET as VCA's have two weak points.
They don't have a large linear operation zone; in fact they are NOT linear, they are exponential in nature so a significant part of the trick of using them as gain control element is to restrict the signal. Obviously, doing so exacerbates noise problems, so the amplifier must have very low noise.
The control voltage feedthrough is quite high, which means that some clever circuitry must be added, that mixes a fraction of the signal with the CV, all this while not impairing the timing characteristics of the rectifier.
In both these respects, they are not much different than the diode-bridge types used in the 2254.
And in addition, FET's must be selected; pairing them for stereo operation can be a very tedious task.
The UREI 1176 adds some trickery by putting the FET in a bridge, which allows somewhat higher gain-reduction for a given CV.
On a different POV, FET compressors are another flavour in the arsenal of studio gear, less predictable than VCA types, different than optos both in timing and colour (distortion) aspects. They are definitely far from neutral; for that reason, they are not one-fits-all and may not be really suitable on some sources.
If I had to have only one compressor, I would take a VCA type, but when you want choice of taste and flavour, one should have an FET type and an opto type at least.
And as always, more than half the performance is in the design, not only in the components.