According to the rejected patch to GCC to support arbitrary linkers [0], mold or any other linker is probably not working very well with GCC. So unless they're wrong, this doesn't really seem like a generic drop-in solution anyway, despite trying to be one.
At the very least, lld is pretty much drop-in. Aside from some subtle differences about resolution order (IIRC lld implicitly has "--start-group"/"--end-group"), everything just works with lld in place of ld.bfd. My recollection was that I had just about the same experience w/gold -- it was just a drop-in replacement.
All of my experience was with using clang as the driver, so I can't speak to why gcc has trouble making a similar feature work like expected.
"Note, all these extra linkers (lld, mold) will not really work properly,
gcc during configuration detects various assembler and linker properties
on which it then relies on and I'm sure neither lld nor mold supports
those features."
[0] https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573833.h...