Look up DNS poisoning, spoofing, ISP manipulation, etc. There have been quite a few attacks on it.

Maybe "flaky" is a bit too strong of a term. It generally works I guess.

The main advantage blockchain offers is not really within the realm of getting the right answer to a DNS query, it's more to do with giving the owner of the domain name 100% control of the domain through cryptography and smart contracts. This also makes the process of domain transfer a lot more efficient, cutting out the need for middlemen like Godaddy, etc.