Is it really open world?

Hardly. There are virtually no exteriors, even interiors used in the story are taken from you. The stadium, the humane labs, the police station, the fbi hq, etc. The map is little more than a cardboard backdrop. It's a pretty backdrop albeit, but completely lacking in functionality.

The missions are filled with mechanics and features not available in regular gameplay, which is completely antithetical to the idea of GTA.

The game won't even allow you to walk and move as you wish since there are 'rails' that force you to go a certain way, most notably on stairs but they're all over the place.

You can't go offroading and enjoy the wilderness and altitude because if you fall from beyond a certain height and can't land on your tires you instantly blow up.

This game instead of being the biggest and most free is by far the most limited and restrictive.