So how much weight can these things hold? I've put some shelves
up using them, and am now concerned they're gonna fall down onto
my head! Been up for a few years now though and don't appear to
have moved.

When used on a wall, the load is transferred into the plasterboard in
compression right down the whole length, which plasterboard is good at
resisting. When used on a ceiling, large bending loads are applied, which
plasterboard is very bad at.

Think of a straight drinking straw cut in half. It can support quite a load
if you stand it up and press carefully right down. However, if you lay it on
its sides, supported at the ends and push down, it has only a tiny fraction
of the strength.

Christian.