It's telling that most instructions deal with the Javascript toolchain and not Rails itself. And let's hope that everything works and none of the 2000 imported node_modules blow up when this tutorial is 3 months old. I'm looking forward to Phoenix 1.6 which ditches Webpack for esbuild. Every step away from the insane churn of the modern frontend world is welcome.
It seems that swc is gaining more traction than esbuild, deno is using it, nextjs hired swc dev and rome is using swc too. Also one limitation by design of esbuild is lack of customization, you cannot hook into esbuild and do your own things, but I read you could with swc. But either way, both are way faster than the most popular ones.
One thing that's important to keep in mind is that esbuild is a bundler, in that it can take an input file that imports other JS files and create an optimized ready-for-production output bundle file. SWC cannot, yet, accomplish this.[1] It is in the works, however! But if I was choosing a replacement for Webpack to use today, I'd definitely go with esbuild.