They totally enclose the rain gutter, so you might as
well not have gutters in a heavy rain when you need
them the most.

I watched a TV show several months ago (either "This Old House Classic"
or "Inside This Old House"). In that show, they were showing a
gutter-cover-type gutter guard that the water was supposed to stay on
the surface of the gutter guard and flow on to the gutter when the
gutter guard surface bended downward. But when they hosed the roof
with water (not a large stream of water), I saw that almost 50% of the
water fell off the gutter guard and onto the ground (instead of falling
into the gutter). The TV show host didn't say anything about this and
continued saying things like the gutter guard was working well. I
found that show was very funny.

I am currently using metal gutter screens in my house. They work quite
well.

Jay Chan