there are several questions in one, but the most simple answer is yes, some daps are far from ideal depending on what headphone you will use. and most of the time you can yourself make an estimation of that by making sure you'll have a good damping ratio.
and also you might wanna check what the amp section can deliver if you plan on using a hard to drive headphone. lot of current for low impedance/high sensitivity phones, lot of volts for high impedance/not so good sensitivity ones, a lot of both for low impedance and low sensitivity headphones. those 3 cases usually answer most questions.
I suppose what is troubling you is to see a BA driver with the same impedance as a fullsize headphone, and wondering why one would be ok and not the other one. well first there is sensitivity to consider. but also the impedance value given for a phone is usually the value at 1khz. that information tells you nothing about the impedance value in other frequencies.
for exemple
the impedance of that multi BA is given as 28ohm @ 1khz
but you can see it changes a lot along frequencies (that is very usual in BA drivers and anything with crossovers). on the other hand, fullsize headphones usually have an impedance response graph that is much more stable(usually, not always). so the value given at 1khz is usually ok to use for fullsize cans.
here for an ok damping ratio, your source doesn't need to be below 28/8= 3.5ohm . instead it should be based on the lowest value so you should seek a source below 20/8=2.5ohm.
any source with an output impedance below 1ohm (as should be for any ok multipurpose sources) will be ok and recommended.
any source with an impedance above, maybe 5ohm will start to have an effect on the signature of the IEM. and the higher the value, the more the frequency response of the IEM will get bent the way the impedance response graph is.
by that I mean in my example, that you will keep sub bass and most of the mids, but between 100hz and 1khz the sound will get slightly lower than it should. and same with the trebles.
the value of the change in signature will mostly depend on the impedance of the source, but with a high impedance source the change in signature could be of several DB.
so as you can guess, if you're gonna ignore the damping ratio(and you never should) it becomes very important to know if the impedance response graph of your headphone is flat or not. if flat you don't care that much, it's not ideal but the signature at least will stay the same. if as above you get spicky variations in impedance, you should avoid it and look for a better source(better as in "with proper impedance for that IEM").
now about why a dap can be said to be bad for IEMs, if it is not because of a high outpout impedance, it's usually because of some audible hiss. as IEMs are very sensitive and BAs even more, it is usually on those IEMs that hissing is very audible. now I don't know the ihifi, so I wouldn't say anything about it. but the part saying it is bad even with an amp sounds wrong to me, as a capable amp will solve both the hissing and the impedance. and from the pics that dap has a line out, so I see no reason why it would be bad with an amp(if it needs one).
