"Some people like NOS DACs, most do not. I would go on a limb and say no one could reliability tell you whether a DAC was R2R or DS without being told."
I am no sure whether r2r vs d-s aren't discernible. The whole idea is, if implemented properly, the technology should actually be transparent to the end user. The dacs should ideally sound the same. But technically or psychoacoustically, they do not. Some devices to sound fatiguing over the long term, this is essentially what all these discussions boil down to. But that's a complex issue. After a hard day of work, I could be fatigued easily listening to music vs a nice pleasant day, in the same system with the same tracks.
NOS vs OS may be far easier to discern. R2R does not have to be NOS, BTW. Most (all?) Schiit dacs are OS r2r dacs. All NOS dacs will attenuate the high frequency by ~3db due to the nature of signal processing. Sinc(x)/x function. By reading many topics over years in this space, I realized even some pro reviewers do not know this. This attenuation doesn't mean treble will sound bad. But treble is definitely rolled off if the DAC is NOS. And you can counter that if you choose to, by using software to oversample prior to sending the signal to the NOS dac. Then the attenuation happens beyond the limit of human hearing. Most of us probably do not have the capability to hear up to 20KHz, last I checked I could discern up to 17KHz. So if the r2r dacs starts to attenuate at 15KHz, this will not be very noticeable as most of our music rarely contains components within the extremities of the bandwidth anyway.
That's also something everyone should test, there are plenty of web based or youtube based signal generators to test your hearing (and your system).
Further reading: https://www.dsprelated.com/showarticle/1191.php
"FPGA is not a type of DAC. It is a method of implementating digital logic and algorithms."
I know that. I meant the class of dacs that use FPGA chips could be called FPGA dacs. PS Audio, Chord Dave etc. The chips can be modified by software. Hence PS Audio dacs are modifiable by software updates.