Ok, so I do not think we will ever reach full concensus, but because I said I would, I went and measured current as well as voltage on a range of devices.
See attached table with AC values. DC was always 0 for all, so I did not even add it to the table, my basis is a 230V 50Hz mains
My conclusion: the FY6600 is indeed not the best (and I focus mostly on current) but in fact the Rigol DS1074Z is a lot worse. If you disconnect the earth.
And if you do connect them both to earth, again the Rigol is worse.
For the rest, the official Samsung USB is the best, by far, followed by my cheapo china linear bench supply.
In fact, the Samsung is so good, I had to doublecheck my DMM had not stopped working.
I measured some more cheapo supplies, but stopped writing them down, most are similar to the FY6600.
What I have learned from this is that if at all possible, I should avoid using my Rigol in "fake differential" mode by disconnecting the earth. I have done that in the past, have a special cable for it. I will modify that cable to also have the Y-cap (actually X2 because I have those) and 1M resistor, and use it sparingly. Yes, go ahead and scold me for it.
If only the math function (A-B) on the rigol was not so annoyingly slow.
The FY6600 will get a X2 and 1M, pretty much like beanflying, but also a switch so that most of the time it is simply fully grounded and not floating at all.