Without knowing what the parent really intended, I assumed they were talking about the difficulty of deploying the same application across platforms rather than with desktop development on any single platform.

Different system libraries, different window management environments and user expectations for layout and interaction, different handling of hi-dpi displays, different filesystems with different filename restrictions and different handling of filename encodings, different application packaging, different code signing and sandboxing requirements, different package distribution mechanisms. I've probably missed a few, but those cover some memorable enough annoyances for me in recent years.

I don't enjoy web development, but I do enjoy web publication and distribution.