Performed a simple upgrade from mysql 5.6 via apt-get on Ubuntu 12.04. Everything worked so far (had to run mysql_upgrade manually though). However, logging is only done to syslog, even though I explicit set variable log_error in my.cnf. Looking at the status output during runtime shows that the variable is empty - seems it is ignored during startup (however other variables from the same my.cnf are working correctly).
For some reason the mysqld_safe script runs with option --skip-log-error, I see that when examining the running process. Looking through the startup script in init.d and mysqld_safe itself I could not locate where this comes from unfortunately.
From reading through the relevant articles on mariadb.com regarding logging etc. this looks like a bug to me. A search on stack overflow showed some users having similar issues: