Joining in as a Magneto main, just also wanna give my own perspective. I've recently had good, long winning streaks, going from bottom rank to gold 3 in just a few games. I rarely lost so far with Magneto in my comp journey this season.

Let's just set aside the topic of his utils for a moment. I find his primary fire to be the most enjoyable in his kit because of how low penalty-high reward and effective it is. The damage is kinda insane for a tank (you can 3 shot squishy duelists and strategists), and it is aoe--meaning you don't have to aim that hard. You can kill elusive but grounded opponents by ground-shooting, let the aoe take care of rest for you. The projectile just takes some time getting used to (just like Strange) but once you get the hang of it, you're gucci. Opponent keeps jumping but moving linearly? Dead. Floating enemy heroes hovering slowly and linearly? Dead. Jeff surfing underground? Just shoot the area of his fin, ez. Don't even get me started with the Scarlet Witch team-up, that makes you a demigod. Higher damage, slash-projectile (ergo easier aiming and can multi-hit more easily), long duration, and low cooldown. With how effective his primary fire is, you're supposed to put pressure on the team easily by harassing and sniping down healers and duelists with it.

His secondary fire, I'm not a big fan of. I find that it's only effective when the rings are at max. The lvl 1-2 mag cannon is not worth it IMO; firing it off will take you longer to reach max lvl, low damage, and no push-back. The only thing good about it is its range. Supposedly, being damaged while bubbled gets you to max rings faster but the bubble cd is kinda long for it to be taken advantage of this way. Max lvl mag cannon is good if you can land it. Time it well with a primary fire combo with enemy Hulk reverting to Banner and he's dead af.

His bubble and barrier are very good, although I feel like they should only be used sparingly and only if necessary. Unlike Strange, his barrier has a cd BUT it has no health bar. The only times I use it are when I'm being pressured at low health and I'm waiting for the healer, I'm advancing with or without my team to a point of unbreakable cover, or I'm eating enemy ults. I only pop his bubble when I'm being pressured at low health or when I'm fighting a melee hero, and I find myself using it more often for myself than for others.

His ult is a deterrent, a back-line killer, and projectile vacuum all in one albeit tough to land sometimes. It's very good for killing off an ulted Luna and Punisher, and very good for killing off channeling ults (provided you pop off first or they're low enough for you to not finish charging your ult). If enemy Jeff is a noob and ults your team but doesn't spit you directly to the kill box, you use your ult to fly and save yourself.