I'm a bit disappointed that this can only be used with NeoVim yet it claims to help you master Vim.

Sorry not meant to be a criticism. Maybe this is the last push for me to switch to using NeoVim.

I waited as well long time, but after switching my tooling and usage has improved a lot since lot of progressive community around modern tools that NVim supports. So I encourage to take a look.

I still keep vim configuration around but I've never felt the need for going back.

I'm making my (probably) 4th attempt in migrating from regular Vim to NeoVim, and while it's better now, the learning curve is still steep as hell. Going the kickstart.nvim route this time and boy, half of the stuff there is pure magic. Honestly feels like reading Nix incantations. Dynamic nature of Lua makes it even harder. Thankfully ChatGPT is pretty good in generating configs.

What's the difficulty to switch from Vim to Neovim?

I switched few years ago, and the switch was instant.

Afaik Neovim is fully backward compatible, unless maybe for some obscure features.

Not the parent, but I also tried to switch around two or three years ago and was very frustrated. The draw was the 'built-in LSP' with all these new features, along with treesitter.

'Built-in LSP' turned out to mean installing and configuring three pretty involved lua plugins at the time iirc. The experience really highlighted for me how un-seamless lua was in the project compared to vimscript. I found it to be a nightmare to configure and get used to. I came away wondering how it could possibly be such a pain, and baffled as to why it was claimed to be 'built-in' when getting ALE to work on vanilla vim took way less time, and felt way more in line with the rest of the program. Ironically it actually gave me an appreciation for vimscript that wasn't there before.

Eventually issues continued to kind of build up for me until I decided to just cut it. Everywhere I encountered lua felt crufty and difficult to work with, and those integrations made the concise, tight vim I'd gotten used to feel really nebulous and unweildy. I never quite got the treesitter syntax highlight to work correctly, and even when it did work having the highlighting dynamically change while typing frustrated me. I ended up slowly switching back to my old vim-compatible plugins one by one, until eventually I just went back to vim, since a lot of the neovim features that diverged from the original design philosophy bugged me and since I'd developed a strong aversion to anything involving lua in the program I wasn't getting anything out of using neovim.

Not a criticism, just noting that it says it helps master Vim motions, not Vim