Phasing can be tricky for several reasons. One of the biggest ones though is that it can lead to a divided playerbase. You could have 20 people in a town in 5 different phases but due to the fact that the players are out of sinc with each other they only see 4 or 5 at the most. This can make the world feel a lot more empty and also cause problems with people playing together if they are 'out of sync' on certain content.

It also requires more server space and processing and tends to have a higher risk of bugs the more its used.

In short, Technology like Phasing is best used in moderation.