I think a full compliment of testing would be required with interfaces to be really useful.
DAC performance is one thing, but if folks want a fantastic USB>unbalanced DAC there is the Topping D10 in a metal case with a nice display and available and fulfilled by Amazon in most countries. Sure it doesn't have a physical volume knob, but cheap analogue ones are crap anyway, and there is no headphone amp, but again, cheap interfaces don't have amazing headphone amps.
The headphone output and DAC performance and notes on the "balanced" outputs if they actually are or not would be good.
With interfaces, the pre-amps are something people generally want to use at some point - else they buy a DAC /+ Heaphone amp.
I think testing ADC and pre-amps thoroughly (channel bleed, strange behaviour, headroom, pads working before the pre-amp etc.) would be time consuming but valuable.
Finally, a test that would not really be so great and easy to do, would be installing the drivers and trying to use the interface over time on Windows 10, Linux and MacOS systems. Some interfaces are stable under one platform but are just useless under another, despite what the manufacturer states as compatibility.
I was going to buy an interface to replace an old PCI card I had in my previous PC. After all the reading and searching and all the moaning I ran into, the only interface that I would seriously consider is the RME Babyface Pro (or old Babyface) or the HDSPe AIO. Cheapest solid and reliable interface is around £600. Low latency, reliable drivers, high quality low noise ADC/DAC sections.
I ended up grabbing the Topping D10 as a way to get audio to my powered monitors. Sounds fantastic! I don't really do much recording, even less without an interface
so a plain DAC does me well for now.
For amusement I was going to buy a Behringer UMC 204HD or 404HD and the cheap XM8500 mic or C1 mic. Just a cheap setup for skype and voice overs etc. Play some guitar straight in DI and use FX 'in the box' (round trip latency might be an issue). But I know I'd end up wanting to measure something and things like channel bleed from Ch2 into Ch1 doesn't seem good. The fact that the outputs aren't properly balanced, and the 1/4" inputs must be balanced, else generate a strong 2nd harmonic(?) don't give me good feelings. It's cheap but it seems a bit like too much got missed.
I know Focusrite put their pads in a funny place on the 6i6 gen.1 - after the pre-amp, but before the ADC. So if the gain control was at minimum and the pad was engaged, you just get a clipped signal 10dB(?) lower than you would normally. A pointless pad. Their 2i2 gen.1 couldn't take much signal into the Hi-Z input.
Seems like a lot of work for Amir to test interfaces thoroughly.
A simple DAC, ADC and headphone test might be too simplistic. I guess ASIO4ALL is what is going to be doing the job too, no driver installs due to effort and faff.