... With GDI, only air passes through the intake valves .....
Not the case, given the recirculated exhaust gas coming in as well. And that's what can cause deposits on the back of the intake valves, because there is no gasoline washing it off, as there is in port injected engines.
The real question is how much of an issue these deposits become over time in the Kia/Hyundai engines. It's well known that there were quite a few problems in the past with a number of Euro vehicles (mainly German), due to excessive valve deposits. Expensive cleaning, such as walnut blasting, was often needed in order to keep those deposits under control. However, it's much less clear (so far) how much of an issue it is on the direct injected Korean engines.
I've seen virtually no accounts of valve deposits causing a specific performance issue on a direct injected Kia/Hyundai engine, and subsequently being resolved via valve cleaning. Not saying it can't be happening, only that I haven't seen it being reported so far.
And another thing that's very unclear is just how effective these various cleaning products are at removing carbon deposits from intake valves. There is one guy on this forum who has reported definite success removing valve deposits, based on borescope observation, using a CRC product that goes directly into the cylinders. Outside of that, I'm not aware of anyone else showing evidence of intake valve carbon being removed by any of these cleaning products (which are typically sprayed into the intake in one way or another).
And one thing that's almost never discussed is the possibility of damaging effects of the cleaning products on the engine. Some percentage of the cleaning agent must be getting into the oil, and eventually come in contact with seals and gaskets. Yes, the amount getting into the oil is likely to be very small, and I have no idea if it could actually cause significant engine damage. I guess I just don't like the idea of dumping something into the engine which is (supposed to be) strong enough to dissolve carbon deposits. Just my uneasy feeling about that, but clearly not supported by any published facts.
Bottom line for me is that, when I'm eventually forced to own a direct injected engine (which won't be a Kia/Hyundai), I will not be doing an valve cleaning process, unless there is a clearly established and documented need to do so.