The KX3 like the FT817 is a nice toy to own because its cheap and affordable.
The base model KX3 is at least $300 more new compared to the base FT-817ND, more if you have Elecraft build it for you.
Obviously if you use a decent antenna like dipoles your success rate improves, which is how most people do their qrp operation.
Preaching to the choir?
While its nice owning a cutesy small radio, the real limitations with these kinds of portable radios is their battery life. In this regard the KX3 is klutz because it has poor battery charge management and poor battery selection options.
WHo cares if you can fit a radio into a wristwatch if you cant operate it for longer than 5 minutes? This essential point is a forgotten by many designers of portable ham equipment.
KX3's current draw is as low as 150mA. 3000mAh NiMH batteries in the battery holder would give you a significant amount of operating time. You may have had a point if you were talking about the 817, though.
Its exactly the reason why I prefer my old AEG HF manpack radio. I can just about use it for a whole day without worrying the batteries going flat. I can connect it to my car battery, plane battery or AC for charging. I carry spare battery module which gives me another 24 hours of operation.
With the KX3 battery module you can do the same thing.
Rechargeables are no fun either, most of the ones that you buy at discount stores are Chinese junk that fail after 3 or 4 charges if you are lucky.
So buy better ones online? There is such a thing as false economy.
All this just equals huge hassles and major annoyance factors. It would have been easier if the KX3 had a build in battery pack and smart charging system with the options of plugging in a Solar panel, 12 to 50 volt DC source or a AC source and all of this in 1 box. These days with portable solar panels that have built in MPPT regulators/chargers you could have been operating and charging the KX3.
Again, KX3 battery module.
Now if you add all these features to the KX3 or FT817 you hardly have a portable solution. You have mess of electronics laying in a big pile thats a nuisance to carry. You would need a pack mule to carry all the options and bits and piecse if you going for a serious trail walk. Now I can only imagine how the mess that you carry worsens when you carry a HF amp. I am not worried about receiver performance I am more worried about my back from all the extra weight I will be carrying with all the required accessories to make the radio effective!
So don't carry an amp? There's a reason why this section is called QRP.
You dont have a portable solution if you cant operate a radio for at least 24 or 48 hours without being a near a recharge facility. When ham radio manufacturers start building a product like this they will get my money, in the meantime I will just continue with my homebrew/surplus junk that does the job in more effective manner like most QRP/portable operators. The KX3 is not silver bullet miracle that solves all the problems and limitations that confront real portable operators.. You cant rest your laurels on receiver performance only! In my world. a self contained HF manpack is the ideal portable HF setup, and 20 watts is more than enough to make the whole exercise enjoyable.
So QRP isn't for you, and that's okay. To me, these all seem like non-issues.