This is a different kind of sandbox. BPF functions by drastically limiting the shape of the control flow graph that will be accepted for a program; most working C functions will not be accepted in a BPF program. The simplest example (complexified by more recent BPF verifier work that relaxes this... somewhat) is that BPF programs can't have loops, at least in the sense that a normal program can.

You can crash a kernel with a BPF program. But it's overwhelmingly likely that the crash will arise from buggy pre-existing kernel code that just hadn't been seriously exercised before eBPF gave people new tools to push that code with. What's much, much less likely to happen is a segfault or NPE in your own BPF code.