The thing with Desolator-items is that they tend to either be very very powerful or very underwhelming - depending on what round you are and how much you dedicate to rush them.
Desolator 1 - 4 can absolutly crush the game during early waves if someone rushes it.
Same thing with divine desolator and later in the game bahamut claw - if someone aquires it asap, it makes some of the next round extremely easy.
Bahamut claw is one way to make NG+ a walk in the park currently.

Its just the way armor works in Dota2 (and by that, in the old warcraft 3 engine)