Being one of those reviewers, maybe I can give you a bit more perspective on the "why". When you review something, you usually want to do so considering the largest possible set of use cases for a product so that you understand how a product might work for as many people as possible. So when I test an amplifier (or a DAC/amp, as in this case), I tend to test it with a wide variety of earphones and headphones in order to cover extremely high and low impedance and sensitivity values. Now, I need to say that an amplifier is too powerful for IEMs if that's the case, otherwise people are going to buy an overpowered amp for their shiny new 8 Ω, 120 dB earphones only to happily have their eardrums blown out (and to then write angry comments like "why didn't you say that it's too powerful for IEMs?").
It's important to judge a product in its entirety and if an amplifier isn't able to drive sensitive earphones at low volume, then it must be stated as such. Also, the problem with the specific situation is that in previous products the feature was available right there without any need to spend further money, so it is clearly a regression. Sure, solutions exist, but it's as if they suddenly removed an accessory from a car that was included in the price in previous models: sure, you can add it, but you have to pay more and that's hardly a great thing.
On top of this, as other people noted there's no alternative for IEMatch if you use the balanced output. What you have to do is to use a 2.5 mm cable for your earphones/headphones, then connect an IEMatch 2.5, then connect an adapter from 2.5 mm to 4.4 mm. I find this rather clunky and inelegant, compared to having a simple switch on the amplifier.
In my opinion it would actually be unfair if reviewers didn't point out this. Our job as reviewers is to hold companies accountable for their choices and give a wider perspective on the market to our readers. So if we abdicate from this role of pointing out (possible) issues, we basically give up on our duty to give our readers fair, balanced information that they can rely on to understand if a product is good for them or not. This is a larger issues with the media industry right now and I could go on writing about it for ages (I am a journalist by profession), but this is to give you the gist of the issue. I hope this helps!