Monoliths and Land Raiders have some pretty big differences for comparative purposes though:
1) Monoliths can deep strike (Teleport) very easily
2) A Monolith is a separate formation to what comes out of it. That has a few important sub points
a) You don't need to choose what is in it at the start of the game
b) The two units can move away from each other freely
c) The Monoliths can double without wasting the assault potential of the troops
d) The Monoliths can lay down pre-assault blast markers
e) A destroyed Monolith kills no transported models
3) Monoliths are fearless. This is a big bonus, even if they can break easily.
4) Troops can use Monolith portals to cross long distances through on-board teleporting (making them much more efficient for time)
5) Each Monolith can 'hold' much more than a Land Raider can in its portal
I like them as-is, but guess our differences are one of expectation- I expect the Monoliths to do nothing more than be very tough and to help with troop transport. If I wanted firepower I'd take a different unit.