Actually, against the monolith, the only way to have killed it would have to rolled two 6s with the VC. Str 6 HT can't do amphetamine parrot against it.
Yes, you CAN ignore something sitting in the middle, when there are bigger threats from units that are actually killable. 4 medium-strength shots (the str 8 ones that cannot do more than glance vehicles) from 300 points is far from scary. Instead, the opponent will be able to focus fire on the easier killed things and wipe them out sooner, and then turn on that monster and whittle him down to nothing.
The main problem with it as a "CC beast" is that not only does it have a fairly low number of attacks (lashwhips are bad against most things a tyrant should be fighting), and moves 6" a turn.
Running 1 synapse at 2000 points with a lot of gaunts isn't crazy; it's just stupid. You then have 24" to deploy your units in on the line, and they have to stay within that range the entire game. Without wings, those hormagaunts will get out of range fast.
I'm not sure how it was "high effective" if you followed the rules correctly. All of those things you popped had only the slightest chance to kill outside of combat (like, 2%), and no competent opponent will let a tank get charged by a walking tyrant.
Or maybe, seeing what your opponents were fielding and using against this unit, maybe you just need to get some good players to play against. I've never seen a good necron player use both a Nightbringer and a Monolith in the same list. Or send the nightbringer against something with a lot of lash whips.
Standing in the middle of a 6x4 table for synapse means that anything charging the opponent will get out of range fairly easily, and you can only contest the middle; the flanks are open.
So, maybe you've been really lucky with this build, but I can tell you that it's far from optimal points spending. And that's what my list and I are all about: spending points in the best way possible.