for its points and reduced protection i was dead against the monolith. But in my 1500 pt list it is doing very well. i only take one ark so my maneuverabily is limited. Combined with orikan i can wait till T3 and drop it away from melta and suck a unit to contest/cap an objective or save them from impending doom. The other day 15 genestealers ignored a flank as my oppo knew i would just suck the unit away to safety. I was able to nuke 2 pallis with the portal and also make them go around the skimmer mountain which really slowed them down. Even vs melta its not a guarranteed destroyed.
It is still very sturdy as its AV14 and not open topped. which is a big thing for our mech.
I have found that if you pick your battles and use it as a strategic weapon that the oppo has to plan for its DSing and its very handy.