It’s all about the player. The character is just a bonus, unless you use VERGIL, that guy is DERP on Crack!
One question, who can combo off their instant overheads on sentinel without an assist?
Everyone… 
Missiles have benefits that Drones don’t, sure. But the downsides are there too.
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Missiles don’t cover any space that you want to use to approach, Drones do. For example, if someone calls Missiles and tries to approach my Nova, say with a Spencer grapple, i can airthrow them for free, block the last couple of missiles if the animation didn’t eat them all and still get a full combo. Many other character can do this, either with air throws or combos that move them forward quickly enough to dodge the missiles as they fall.
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A lot of characters can call Sentinel on first getting the opponent to block something and maintain blockstun long enough for the Drones to hold them in place for a mixup. Not many can do this with Missiles, if any.
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Some may view Missiles tracking as a plus, but in a lot of cases having an assist that will always occupy a vital area of the screen, ground to normal jump height, is more powerful than an assist that you can lure away from danger, or if you move fast enough evade completely.
Plus the stuff i mentioned earlier.
I don’t disagree that as the game gets more and more fleshed out, Sentinel will become less and less common, but i don’t think that’s because Drones is getting weaker as an assist, it’s because it’s only a little better than other assists that are attached to characters who are a LOT better.
Amazing assist, decent character, ASS HEALTH, Shitty Hurtbox (Instant overheads anyone?). The thing is, you have to turn his weaknesses into strengths!
How do you turn a huge ass hitbox and low life into a strength?
… you use him on point and let him die. that wayyou get XF2 in a “reverse phoenix” strategy.
I read that Jam Session is not a game changing assist, obviuosly some people haven’t played against Zero with that assist;
Aside from that i don’t think Sentinel sucks, has one of the top 5 assist(Jam Session, Tatsu, Drones, Vajra, Missiles), has projectiles and a beam, but IMO the only position he really deserves is the 2nd one, if you put him on point you are not using his assist, and lets face it, almost all who plays this big guy is because of his godlike assist, and on anchor spot it awful, all anchor characters in the game are bad matchups for him, and he doesn’t have the comeback factor.