It's over-rated.
5 minutes to defeat SM Hashirama = must be on drugs. :|
Who said anything about kamui only working for 5 minutes? Is there some sort of cooldown I'm missing?
Obito can remain intangible for 5 straight minutes, after which he must deactivate said intangibility. After he deactivates it he can then reactivate it for another 5 minutes.
That's why konan needed 5+ minutes of continuous explosions & that's why she planted them in the ocean, because obito would a) have to become tangible at some point & b) be falling deeper & deeper into traps, with no ground to phase into.
Obito beats hashirama without info, because he has no attacks that can do what konans did [which is the only way to counter kamui without s/t].