While i agree on the fact that Nagios needs a lot of studying in the beginning, once you've grasped its basics it becomes an extremely powerful and versatile tool, you'll be blown away by the level of customization you can achieve, your imagination is the only limit to what you could monitor
I have tried thruk, op5, opsview, naemon.. they're all pretty much the same, you can't expect to have a monitoring program to already do all you need it to do, because well.. everything you are going to monitor is going to behave differently based on how you configured it and on what parts of it you need to be monitored, and there are so many different products out there that is going to be impossible to maintain a single monitoring suite that covers them all out of the box
The real power of Nagios is in its customization capabilities and plugins, there is plenty of documentation around about pretty much anything you can find in Nagios, Adagios immensely simplifies Nagios configuration and usage, but only once you've actually understood how Nagios itself works
About writing new plugins, that too can be time consuming but again, once you've understood how plugins work and how they interact with Nagios (exit codes), it becomes really intuitive to make new ones
Moreover, studying how Nagios works also has the added bonus of understanding how the stuff you're going to monitor exactly works
The Nagios you find in NS, (and thus, adagios) can be already expanded to send you an email when free disk space goes lower than a custom threshold, or send a message to a slack/mattermost channel if an authorized access attempt is found in your logfile, or send a message on whatsapp/SMS if a service running on a server goes down..
If you'd like to dig into the very deep Nagios' rabbit hole, you can start off by reading how Nagios works here
I would suggest starting off by reading the docs there, you'll find all you need to start using and customizing Nagios, but it is going to take some time.
Once you've got how to monitor something on the same machine Adagios is installed on, you can then proceed to monitor stuff on a remote machine, through NRPE (Nagios Remote Plugin Executor)
If i can be of help, feel free to ask and i'll gladly lend a hand if it's something that can be done, but please be as specific as possible on what you need to monitor