Cedar was a programming language and a development environment (not an operating system) and also run on Solaris as demonstrated in the demo, and I didn't see it crash during the two hour demo.
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Not exactly. The three main environments (Smalltalk, Interlisp-D and Cedar) took over the machine completely. Each had its own network stack, window system, et al. Those environments didn't have memory protection so from an OS perspective they were more like classic MacOS rather than current MacOS.
They weren't general purpose OSes: there was a single "environment" running bare on the machine. That's why porting Cedar, Symbolics images etc is possible: you stick the whole thing in a single address space in user space.
Pilot would crash with probability 0.9^p, where p is the number of things you did at once, p=4 was a limit I never surpassed. With UNIX p=30 was commonplace.
It is easy not to crash, when everything that one is able to do are text based applications.