Zabbix server should take care of time it spent to save data into DB. It happens sometimes that mysql is slow because many items in histrory/trends. Zabbix should take care of this and be able to do some alerting based on this. It is really hard for user to find why monitoring does not work or why only part of datas are in history. It would be enaught to make housekeeper some intelligent work to check if queue was run in all or not. It could solve problem even with many unreachable hosts. Maybe to create some trigger using zabbix[queue] internal parameter and to put it with some triggers to basic zabbix templates?

In fact, there should be kind of errors (like this, database connection problems or server is not runing) which should have absolutely precedence in zabbix and should be visible just after login (maybe some popup window) ?