A senior engineer that has always worked in the backend won't be able to be up to speed with a complex frontend app quickly and be as effective as a senior frontend.
There's just too much to know about browsers and underlying tech and a myriad it things.
Same for a senior frontend starting to work on a complex backend all with god knows how many interconnected technologies.
Being able to go from JS to TS? Sure. Being able to go from JS to Rust + Docker + (all your backend stuff here) not so much.
A senior engineer that has always worked in the backend won't be able to be up to speed with a complex frontend app quickly and be as effective as a senior frontend.
There's just too much to know about browsers and underlying tech and a myriad it things.
Same for a senior frontend starting to work on a complex backend all with god knows how many interconnected technologies.
Being able to go from JS to TS? Sure. Being able to go from JS to Rust + Docker + (all your backend stuff here) not so much.