Nowadays, GTK is designed with GNOME, and only GNOME, in mind. Non-GNOME applications which attempt to utilize it suffer as a result. Pantheon is no exception.

Which may turn quite a few people off with the aesthetics, of course for some people, this is a pro.

Any significant updates come with new release, which usually takes 2-3 years. Hence gets obsolete with current software.

Good for people who want a plug and play simple Desktop Environment. But for a multitasking / entertainment / cutting edge or Gaming Rig this is not the answer.

It is pretty bare-bones on a stock install, but further configuration options can be added through various official tweak tools.

As it has no set release date, and no set support and/or EOL dates, that makes it a not so good choice for enterprises managing a number of machines where such dates are important to know before hand.