I happily used Karabiner for years but only to handle my caps lock remapping. It's not bad. I just generally like having as few dependencies as possible. I definitely needed Hammerspoon for other stuff, so if it can cover my one use case for Karabiner as well, then I only need to use one program instead of two.
One specific reason I want to go with Hammerspoon in the long run for key remapping is that I plan to do more complicated things, like setting up key layers. And even if that's possible with Karabiner, I'd rather maintain the complexity with Lua code rather than Karabiner's JSON configuration, which is complicated enough that someone created a separate tool[1] just to generate the configuration.
I should note that one of the maintainers of Hammerspoon specifically recommended[2] using Karabiner instead of Hammerspoon for key remapping because Karabiner was built to do that. Hammerspoon has a much higher level view of key events, from what I understand. I don't plan to do any remapping with modifier keys though. I plan to only use the letter keys for everything. Something like: press and hold ;, then press and hold v, and then tap a. And that will re-arrange my current windows into my preferred arrangement. Stuff like that. Based on my initial experience, I should be able to do everything with just Hammerspoon.
[1]: https://github.com/yqrashawn/GokuRakuJoudo
[2]: https://github.com/Hammerspoon/hammerspoon/issues/1025#issue...