It isn't so much that as it is that the Xcom units are sort of no longer similar to a specialist team and are more Grunt-like... like Space Marines.
Medium sized aliens that can walk through full on gunfire when you have "The Gunner" would be odd in a Space Marine squad.
In the original XCom, I sent in 8-12 guys and deployed them either in squads of 4 or in a line which swept the map.
Early in the game I would equip virtually everyone with heavy cannons or automatic cannons with a mix of HE ammo and incendiary ammo (since some aliens are more vulnerable to one than the other, but in practice HE worked fairly well and I mostly used that). Did any aliens survive that? Not really. It saved a lot of XCom lives, and the only problem was using it during terror missions (and preservation of UFO resources, of course).
Later on everyone would have laser rifles or heavy plasma weapons, once those were available, along with armor, but my point is that everyone was "The Gunner" (if I understood your meaning correctly), everyone was the same class, using the same weapons, and everyone was using overwhelming firepower. It would be like if SG-1 went out with 2 other SG teams on all their missions, all equipped with something more destructive than grenade launchers, and able to fire more rapidly - instead of rifles.
I don't really see how you get 'space marines' from the game having "scary aliens." If you don't think the chrysalid, cyberdisk, or psionic aliens were scary in the original game, I'm a bit puzzled.
I don't have any issue with the game limiting you to what I'd consider one squad (or one large squad of more than 4 people) instead of 3 squads, or much of an issue with the classes - maybe it will actually be worth it to have variety in the squads, who knows. Or maybe not and everyone will all be one class still.
(Sectoids were dangerous at the very beginning if your troops were dumb or unlucky, or badly equipped, or if they refused to come out of their UFO, etc, as they were capable of killing unarmored rookies with crappy weapons, despite being weak. Even if you hire and fire a ridiculous number of troops and select only the ones with the best stats for your squads. Lack of armor is a killer.)