How effective is ELK than other Monitoring tools?
ELK is pretty ineffective as a monitoring tool in a default configuration as it doesn't do monitoring - only reporting. ELK can only provide monitoring and alerting abilities if you configure additional add-ons such as ElastAlert.

Then, you are impacted by your ability to log errors via syslog. If messages come in via, say, SNMP traps, you will be unable to alert on these if you do not configure the appropriate plugin.

Like most monitoring systems, your effectiveness is determined mostly by 1) your input method and 2) if you have told the system to alert off of all of the correct messages.