kiklo is correct that in theory the lesser PoW chains and especially those without ASICs in theory need checkpoints, but that doesn't even protect them from rented hashrate attacks. However his point does not apply to Bitcoin. Bitcoin has checkpoints to be extremely paranoid such as if for example there was breakage of SHA256 such as a quantum computer attack such as the one described at the end of Iota's Tangle white paper.
Here is theory for you to tangle with.
Quantum Computer creates a Virtual System , that has time emulated where 1 nanosecond emulates 1 normal second in the real world.
Inside this virtual system the time is set back to a month before btc was created.
Using the Quantum computer abilities, it creates a PoW blockchain that is longer with more difficulty in a fraction of the time.
* And the only thing that stops it from overwriting the Bitcoin PoW blockchain are those little checkpoints. *
PoW or PoS face many of the same dangers.
