QSK, or “full break-in”, is more than whether antenna and linear keying relays are quiet. The bigger issue is your side tone sounding like a station sending on the band without being accompanied by audible thumps or other extraneous artefacts. In addition, the radio needs to switch fast enough to hear a station between characters, ideally between dots, to more than 20 wpm.
SDR type technology doesn’t readily switch quickly due to inherent latency in the processing of the data stream. Another complication is synthesiser settling time if you operate split frequencies or use RIT. Add in the fact that few amateurs use Morse and even fewer operate moderately fast Morse with QSK, you will see why the ability to cleanly and silently operate QSK is well down the list of design priorities of most manufacturers.
Sadly, QSK smoothness and speed is rarely mentioned in reviews and is not included in the list by Rob Sherwood. A few years ago Amtor switching times were included in some reviews, these days very few of us use Amtor modems. My IC-7300 works well on Amtor with a KAM Plus modem, as does a K3 and a Kenwood TS-480sat. Amtor requires a switching time of around 20 mS or less, although it is not quite the same as CW QSK… another column for your list!
Give me a few days and I will test the radios I currently have available. I have a K3, with the later “K3S” synth boards, an IC-7300, plus a K1, K2 and KX3. I have a KPA500 and a Hardrock 50 amp too. All, except the Icom, are diode antenna change over.
73 Dave