iMac hangs trying to restart or shutdown 10.13.3
I had the same issue. In my case I was able to see in log that there was problem with cache files permissions. I removed them and now I'm able to install update and restart MacBook without issue.

sudo rm -rf /var/folders/zz/zy*
sudo shutdown -r now


NOTE: some of cache files ware skipped due to system integrity protection - Operation not permitted