Sentries are slower in tracking targets and the arc in which it can aim upwards seems reduced compared to sentries in GOW4. I find they do tend to work, but not really as a true defence against Sentinels and Guardians - they tend to be support - purely to hit them and prevent them from regenerating health.
Sentinels and Guardian’s AI seems greatly improved as well, so they fly straight in very aggressively and move toward blindspots, and because the sentries’ target tracking is weaker they often can’t keep up with moving targets. They also seem to wait for opportunities to attack, so sometimes will just linger around on the outskirts of your base. I think this is why they seem much more potent than previously.
I strongly advise that someone on the team is assigned to Cryo-Cannon duty. The freeze effects mean that enemies take more damage, and with fliers in particular you slow their movement and can even stop them dead in the air. Alternative use Clayton and his Concussive Explosions card.