I think that these are probably the most reliable component of the entire shuttle ... these are ridiculously simple in construction and once lit - stay lit.
I mentioned the SRBs for illustrative reasons. The cleavis joint has a much better design now. The previous flawed design was the main cause for the Challenger tragedy. That flawed design was not subtle. It was a botch job and lots of people knew it yet nothing was done until the Shuttle exploded.
If you think that the solid rocket motors are simple and safe you're wrong.