So many compromises against IoT devices or network equipment come from inspecting the open/recycled firmware.
There are a lot of interesting defcon videos about using this method
edit: To clarify, this isn't to imply that I think closed source would necessarily be better.
I think this quote puts it well. An all-too-common story about security in open source... of four people: Everybody, Somebody, Anybody, and Nobody:
"Anybody could have done it, but Nobody did it. Somebody got angry about that, because it was Everybody's job. Everybody thought Anybody could do it but Nobody realized that Everybody wouldn't do it.
It ended up that Everybody blamed Somebody when Nobody did what Anybody could have done."