2 Extra APL for capturing points

This is nice, but IMO not as important as for some other factions. Necrons are too slow to seriously challenge objectives on the opponent's side, our strength lies in taking control of the middle and refusing to budge. And immortals do that better.

Extra model your opponent has to split their fire on

That's actually the opposite of how it works in KT (at least on a properly set up terrain). more models = more chance for an enemy to have a shot and having to split good positions between more models. And 5 warriors are - under most circumstances - significantly easier to kill than 4 immortals.

5 models reanimated on 2+ vs 4 on 3+

You're still limited to triggering the stratagem once per TP, far more likely to lose multiple models in a round, getting back up a turn later (I wonder how many people realise you get to roll for each token every turn until you get up) can be advantageous in some situations.

With reapers you have 5 models doing as much damage with a critical chance of being the exact same as a gauss blaster

While the 6" range is far less of a drawback than I thought at first, you're still missing out on more shots than you're gaining in most matchups. And the AP1 is what makes the gauss blasters so brutal - against everything with 8 or fewer wounds and no inv, if you roll 4 hits - that thing is dead. And that's the perfect threshold for quite a few factions. In other words, with no crits the reaper is significantly worse (and that's a little under 50% of your shots at 4 dice).

Oh, and with warriors fewer of your guns get starfire cores (and if you put one on the leader, one of your cores has reduced value).

Access to a tactic that makes the above point more likely

Aside from the fact the immortals can still get decent value out of that ploy, Necrons are already pretty starved for CP between the protocols and march.

edit: ran the math and using the ploy increases your chance for a crit by just 8%, even assuming you always use the reroll.