Although I wouldn't recommend using S(ync) as the third option for rebooting the system, after terminating processes etc. If the system's becoming unstable, syncing the drives is the very first thing I'd want to do. I prefer the order S-E-I-U-B. AFAIA, an S before U is redundant as buffers are written out as part of the remount-ro process, so a seperate sync() isn't needed (if anyone knows otherwise please correct me).
But of all those, the U is the most important, as if it succeeds it will protect your filesystem. You may end up with some left over temp files as tasks that didn't receive the terminate request signal didn't clean up after themselves, but this is usually not too great a cost.