I've had all of the detectors you noted, so I'll share my perspective.
First off, I sold off my Red Racer, spare Racer 2 (but kept one) and Impact. The Multi Kruzer made the Impact redundant, and personally I'm not a fan of the ergonomics on the Impact (my thumb would frequently go numb after a day of detecting with it).
Sans the additional Impact modes which I never used, I've found the MMK to be everything the Impact is and more. I find that they cleaned up the audio, it's less artificial and digital sounding on the MMK vs the Impact. The 4-tone mode is definitely superior on the MMK vs the Impact, both in depth and unmasking capabilities. It's very sparky, potentially noisy, and is more susceptible to EMI. For remote sites I haven't found the EMI to be an issue, and I can run it at 98 sensitivity. The ability to run it hot has resulted in some deep finds (like an 11" deep two reale that all my previous detectors missed, and my die hard Exp2 buddy also missed). I found for inner city use, 4-tone may get blown out by EMI. The 3-tone mode is interesting because they incorporated a hyper-speed reactivity when using a sensitivity setting of 89 or less. This comes with a cost of course (there's no free lunch in detecting), and that cost is reduced depth. For hunting sea of nail or ultra trashy sites where depth isn't the goal, but cutting through the iron or trash is, this works very well. The DEEP mode works well, it's basically similar to the BP/boost process mode on the F75 series. Beach mode is similar to previous Mak/Nok detectors (best out there for VLF, but not, IMO, as good as SMF machines).
There's a few things I don't like about the MMK - the button pad is a hot mess IMO. Cannot tell you how much I miss the trigger switch that my Racers and Impact had, it simply becomes an extension of your hand allowing for quick target pinpointing and ground balancing. The pin-point button on the MMK doesn't even have a raised dot so your finger can locate it, you basically have to look at the button pad. The buttons are black, the detector is black, have fun doing this if you night hunt at all, it really sucks. None of the keys are back-lit, so it's challenging to make adjustments in the dark without firing up your flashlight. It may or may not sound like a big deal, but it slows you down, that means your digging less targets.
In a nut shell, I do not one bit miss my Impact. The Impact is also getting less support it appears as their new wireless pinpointer only works with the Kruzers and newer machines. Software updates seem to have favored the Kruzers, as has wireless headphones. Except for highly mineralized soil, the Multi Kruzer is my go to machine for relic hunting, and I can't wait to see what the good folks at Mak/Nok produce as their first SMF detector enteres the market.
