So we're just going to ignore the reapers other fire mode that is better against vehicles then? Sorry, same S in one fire mode but not always. Not the same AP or damage (I'm not sure where you're looking). Either way, your comparison is way off.
Reaper low Strength fire mode is the same strength as the Immortal and the same AP, yes less damage, but you get 2 for the price of one Reaper
It's not a great option but its one of the better all round options Necron have. Lychguard is still one of the cheapest ways to remove a tank in 1 round though, even when you add the cost of the transport, but once youve smashed 1 tank the othercan jsut avoid them, so that leaves C'tan's which are going to be a fire magnet.
Basically GW wants you to Horde it up. Fantastic against infantry, but umm yeah, not so great against tank swarms. Hopefully GW balances that out a bit with the codex... or prevents tank swarms.. or both..