Even Tin T2 which is 16 ohm iem, almost no different with higher output impedanceUnless you have low ohm headphones, the 10 ohm output resistance does not influence adversely the headphone output. As such, it's just a matter of making an informed buy. I wanted to use this with Sennheiser HD800, which are 300 ohms. As such the 10 ohm output resistance is pretty much irrelevant. People should do their homeworks before buying and then complaining (even more so when specs are widely available and published by the company itself). It's as simple as that.
KZ ZST which is 10 ohm iem, doesn't play well with higher output impedance
Each headphone react differently to higher output. As you said, do homework before buying.
Reference Audio Analyzer site is great place to check if higher output impedance will affect a headphone.
I think how the manufacturer implement or create the driver play a big part on whether the headphone will be affected by higher output impedance. Browsing few models, I noticed Audio-Technica have no issue while each Grado's model will have slight hump around 100mhz