Halo 5 actually is shorter than 3 on normal. Except maybe you search for really everything. But on legendary?
It took me over four hours yesterday to fight the three warden eternals at once.
The other encounters where pretty short if you know how to though.
While i don't like the team AI, i remember many gamers complaining about Gears 3 because of the team AI doing to much even on highest difficulty.
So while i don't want the AI to do the whole job, it's outright dumb in Halo 5.