One of the weaknesses of bitcoin listed on that wiki is that it was once vulnerable to the time warp attack. This attack lets a miner with >50% of the hash power to solve many more blocks in a short amount of time, essentially gaining a higher than appropriate reward. It can also be used to create a chain that is extremely long very quickly while keeping the difficulty low.
Was the time warp attack the only reason it was not safe to compare chains by length? Asked a different way, if the time warp bug was fixed in bitcoin, would comparing chains by length always produce the same result as comparing by work done?
Relevant: Theymos suggests in this answer:
Satoshi didn't initially realize that choosing the correct chain by just counting blocks allows for some extremely easy attacks. Version 0.1 just counted blocks. That's why the paper just says "longest". The idea of "chain work" was added a little later.