Since VCAs are quite transparent, beginners feel the need to boost them in order to hear the compression. While VCAs are truly transparent at moderate levels, they tend to grab at your signal once you boost them. Sounding aggressive in chunks, they tend to upset the continuity of the contours of the amplitude of your signal. VCAs also tend to pump in the lower frequencies, when you dial in too much compression. As we saw in our sound examples, the API 2500 has an additional thrust control to handle this low-frequency pump.  Due to this, the disadvantage is that VCAs cannot be pushed as much as FET.  However, since VCAs are extremely cheap to build and provide predictable and repeatable results, their negatives are often overlooked.