I feel that frontend has a lot more quirks than backend development has simply because of how hard it is to get UX right. Browsers are a lot better these days about standards but there is still inconsistency, especially if you want to target certain segments of the population like IE or Safari. Accessibility has general guidelines but YMMV with different screenreaders. And developers are not in the position to ask users to switch to a preferred compatible system just for their special snowflake shopping site or gig economy startup or CRUD app.

In the general sense I don't think frontend or backend web development is more difficult than the other. It has more to do with the problem being solved and the expertise of the individuals involved.

For example, your comment made me wonder what I would point out as example of maximum complexity in either field -- distributed systems, tight performance constraints, exceptionally magical functionality, and so on. Everything I thought of is something that's done in both backend and frontend systems.

I do feel that the industry perception is that backend development is more technical. But I don't believe that is true anymore personally, and I think that perception is rapidly changing.

For what it's worth, I'm much more of a backend developer myself.