Is difficult hashing algorithm necessary? To avoid GPU/ASIC mining, difficult hashing functions, such as scrypt and scrypt-jane, are proposed as PoW algorithm (However, GPU and ASIC are also suitable for basic scrypt operation). Is it good for the cryptocurrency? Because a difficult algorithm also means CPU needs to spend much more time to find the solution. Although the "difficulty" of the cryptocurrency is dynamic and adjustable, maybe one day a very complicated algorithm scrypt-xXx is proposed, it might be hard to find the nonce even when the difficulty is very low. Will it create some problems to the cryptocurrency?
scrypt and scrypt-jane were chosen not because thay are "more difficult" then SHA-256, they are just different, and less suitable for GPU/ASIC implementation.

Whether using such algorithms is good or not is hard to tell. I'd say, since one can mine scrypt coins with commodity hardware now, while mining SHA256 coins even with decent GPU rig is futile, scrypt helps coin be more decentralized, which is a good thing.

One can of course create very difficult hashing algorithm (I'd say 10^80 rounds of bcrypt will be enough to ensure no one calculates a single hash ever). But cryptocurrency with such algorithm is unusable, and so would be forfeited.