[speby]

This is itself too much of a generalization. Great developers with a lot of experience across tech stacks, even if mostly back-end focused, will have very little problem getting up to speed on a complex (e.g. Angular) front-end. Learning curve? Sure, but these are the people that have proven time and again they have no problem and no fear picking up new tech stacks and applying their general problem solving skills across a huge variety of languages, frameworks, etc.

A more concerted challenge is not whether a senior developer can pick up front-end technologies, but rather whether a strongly backend focused engineers wants to do front-end. I find the latter scenario (which is more about desire and preferences) to be far more important than whether or not they have the ability, experience and aptitude to do so.

[logfromblammo]

As one of those back-end-preferring developers, the thing about front-end development is that the correctness of UI code is subjective, and furthermore subject to the user's opinion. On the back end, I can write heaping piles of unit tests to verify objective correctness of math and data consistency. But on the front end, this button needs to be further to the right, and labeled in a different font.