The CS43131 has a current limiter feature, 120mA I believe. If this current limit is the total for both outputs, a 4 Ohm IEM may hit that limit.I asked customer service whether it pairs well with a 4ohms IEM (Elysian Diva), and they said no. So I guess either the OI is not so small or they simply don't know.
In fact, they cannot recommend any of their dongles for my Diva (I appreciate their honesty tho