Personally I feel like Makoa kicks ass as a sort of "secondary" frontline, ie having another frontline on the team, most likely Fernando. He can sit on the point for ages, sure, but he can't really threaten anyone while doing it--he can't attack while doing his shield thing, his ult is, well, an ult, his pull is basically reliant on teammates to be effective and, if he's on the point already, he probably needs to focus more on surviving, and his basics don't deal enough damage to really confront an enemy frontline imo. However, he complements Fernando and/or Barik wonderfully; in an even exchange, shielding or adding damage to the enemy frontline can help remove them more quickly while not having to worry too much about staying alive thanks to his huge healthbar (and shielding the squishier Barik or Ruckus would be helpful and prevent them from collapsing).
Fernando, on the other hand, has a very strong defense in the form of his shield, while having respectable damage output with his basics and fireblast, ESPECIALLY if the enemyteam decides to line up. His ult is much less useful, but can still help in emergencies I suppose. His health isn't as overwhelming as Makoa, but still respectable.
Ruckus is just too squishy to be helpful as the only frontline, in my opinion. Barik can get away with his low health thanks to his shields and high damage potential, but Ruckus, even with his right-click and Q activated, just folds too quickly when focused. He has much higher damage potential than Fernando or Makoa, and maybe Barik, and his dash is objectively bettre than his competitor's, but he will inevitably be foced off the point either by death or bullying much earlier than the enemy's frontline, and that's always a great way to lose a point.
EDIT: Also, I wonder, what role do you guys think is the most helpful to double up in? I haven't played enough to be totally certain, but I'm kind of leaning towards frontline. A damage going down hurts, a flank less so, a support dying will suck but can be weathered. However, if you lose your frontline while the enemy's is still strong enough to sit on the point, it seems hard to really hold it, and once they're entrenched, it'll prove kinda difficult for the frontline to get back on the point and stay there. Also, as I talked about above, I feel like the frontlines support one another better than the other roles.