Government auditor for NASA contracts here, fixed price contracts aren’t perfect either - they can unfortunately incentivize contractors to de-prioritize some fixed price projects in favor of others, especially when there are delay issues (supply chain, COVID, etc.) This can cause schedule delays on the NASA side that translate to gargantuan cost growth for projects, or even cause them to miss planetary launch windows that might only come up every couple years or so. They might be arguably the least bad contract setup, but they have plenty of issues. I’ve spoken with project managers at the agency who prefer cost plus because they can let contractors use things like double shifts and overtime to compensate for these delays and pass the cost on the the agency, something they would never do under fixed price.