Well you can apply damage while harpooning, for one thing. Griffin has to choose between firing his smg and having his harpoon gun out, waiting for an opportune moment. When he’s doing that, he’s neither applying cc or damage. With Maggie, you can do both.
She also is much better at keeping people alive, as her harpoons don’t have as much downtime since there are three of them. Another advantage of Maggie over Griffin is ease of use. Griffin has to be pretty damn accurate on the harpoon gun, and has to wait for good moments to use it, then suffer a long reload time.
That’s not to say Griffin is useless, though. He provides decent survivability to his teammates in dome, asking with more control than Maggie. His biggest strength, though, comes after the dome drops and the monster tries to flee. The range of his harpoon allows him to keep snagging the monster as he tries to run, granting indispensable chip damage. So they both have their uses!