I was glad to discover Havit's line of low(er)-profile mechanical keyboards, sating, for now, my desire for a flat keyboard with fast and clear tactile feedback. I bought the 87-key KB390L for myself and a 104-key KB395L for my wife. Her experience has been good and we might buy another for her office, but I've had some issues worth describing.
First, my KB390L had an intermittent 'f' key. Like, 10% of the time hitting 'f' resulted in an 'f' appearing. Havit sent a replacement promptly, but the replacement had a non-functional Enter key, and that's a show-stopper. But having worked two weeks with an intermittent 'f', I decided to 'fix' both keyboards.
I was unable to buy new switches, the Kalih Blue low-profile are too new/low-volume to be on amazon/elsewhere.
So, I made a key-cap puller from some wire and followed this disassembly guide, managing to avoid breaking anything in the 'repair'. Inspecting the Enter's switch, the leaf-spring contact appeared to be incorrectly bent, causing the contact to not close. The 'f' key's failure wasn't obvious; these switches are quite simple but thankful able to be disassembled for those nerds that want to change the plunger lubricant or spring.. 'Repair' is in scare-quotes above because I ended up swapping the affected switches for keys I don't use, namely 'f' for 'Ins' (which I NEVER intend to strike) and Enter for, I think, Scroll-lock. So far, so good.
With the keys largely fixed, I'm annoyed by two aspects of the firmware/driver. First, some keys are programmable, but you can't program volume adjustment keys into the 390L. On my 87-key keyboard there are 22 keys dedicated to changing the key light patterns, but none for the very basic task of changing the system volume/media controls.
Second, at least on Linux Mint, I had to mess with the usb device power states to get keyboard to be able to wake the computer from suspend, and once awoken I have to unplug/plug-in the keyboard to be able to log in. I definitely don't know what's going on here but I just expected it to work like every other keyboard.
So, I wouldn't recommend Havit's keyboards with the Kalih Blue low-profile switches, unless you have a soldering iron and patience. That said, I hope Havit has proved the market and that they will improve the quality control or that the bigger brands will offer lower-profile keyboards.
With all of that said, as weird as it may seem, I do enjoy typing more on a mechanical keyboard, and the muscle-memory combos in vim are even more satisfying than before.