I see it as the biggest issue in C++ for large projects.
The separation of header and body is purely to "make the tool chain happy". A typical header contains both interface and implementation details - to avoid that I have to write more code. The separaton is awkward from a software design POV. Header-only code lacks certain features (static variables), and is suitable only for stable libraries, as incremental build times usually suffer massively.
#include's go stale after ever so many refactorings, unused #includes are hard to identify, creating a spaghetti bowl of fake dependencies that are hard to understand and harm incremental build times - which in turn harms the entiredevelopment process. Slow turnaround means slow tests means tests are run less often. Large projects often suffer from bugs / shortcomings of the dependency check, triggering full builds "just to be sure".
Moooar hardware, forward declarations, precompiled headers and PIMPL can mitigate the problems only up to a point. The latter makes me particulary uneasy: solving a tool chain problem by changing the way I write code.