The backend has plenty of complexities, but frontend developers have to deal with something just as complex - the user.

Given ramp up time, most backend engineers could build a bad frontend, or build a good one if they have a really good UX team that thought through everything and are just implementing their work.

In the real world though where UX is understaffed and often focused on the wrong problems - I've had to rescue too many frontends built by backend focused teams to share your confidence.