Bad shield, can be poked super easy, might be possible to even poke him at full
Bad rolls + other OoS options are average or non-existent
No good anti-air options
Bad ground speed/DD
Generally slow/low priority aerials and slow jumpstart/aerial drifting/fast fall speed
Best defensive option is CC dsmash, can be escaped for frame advantage on the opponent's part reliably without good reaction time, just with MU knowledge to know when he can CC and thus when to tap up to escape dsmash on an approach. Alternatively good DI away can often just reset the situation for the opponent, maybe giving ROB more stage control but it's not a whole lot for trading damage and winning.
His projectile camping options have heavy cool down between uses, and one of them can be stolen from him. But he needs to use them so he can get the opponent past CC percent for boost-> fair so that he has something to approach with. Plus Powershielding is a thing so any character with a reliable shield and a player behind them with practice PSing ROBs projectiles, or just any character with a reflector, is going to do a good job of shutting down ROB's camping game. Plus laser is super telegraphed so you have to read where they try and avoid it. Just try to out camp a good fox SHDlasering. You figure a character whose primary strength Is having two specials devoted to projectile camping would be great at shutting that down, but nope, he'll just SH over gyro and/or laser. Or you can try aiming laser up to snipe the SH at its peak, but the hitbox is small and isn't in any one place for more than 1-2 frames. By the time you've hit him, he's probably tacked more on you. And he can mix up shine to reflect of course.
His combos generally rely on bad DI (fair/upair strings on most characters [nobody seems to DI these right often but when they do they get out immediately] or dsmash/nair into grab on fast fallers [DI away prevents this at most percents]), or rely on his dthrow, which is very good. But he isn't fast enough to abuse his above average grab reach. Fortunately boost->fair/upair lead to grab pretty reliable unless they CC, which is why you should camp them until you can beat CC; see above about projectiles.
I don't think it's so much that he's bad, just outclassed. Samus does almost everything ROB does better.
CC? Check, she's heavier, has two CC options, not just one, that cause knockdown immediately, and neither loses to preemptive SDI.
Poking options? Her Ftilt and fsmash both reach farther, come out faster, do more damage than ROB's ftilt/fsmash, while still causing knockdown at the same percents, still being aimable, and still being safe on shield. And you don't have to sweetspot for max damage.
Airdash? She has zair. It doesn't lead into follow-ups as well, granted, but it covers the same area safer than ROB can with airdash.
Camping options? Her missiles pack more knockback, have less end lag and less cool-down between repeated uses, unless she doesn't have platforms. Plus she has a long-range early-kill move in charge shot. She can kill without almost ever leaving neutral.
OoS options? Not even close. Samus has better rolls, a godlike WD, and an actual up-b OoS. And a better shield too.
Ease of escaping combos/juggles? She's slightly floatier, and has a sex-kick Nair that comes out faster than ROB's fastest aerial. ROB's combo escape options are: mix-up boosts/DJing and hope the opponent isn't fast enough.
Anti-airs? In addition to better fundamentals like DDing and WD back for good spacing (generally good for beating SH approaches), she has utilt, ice fair and to a lesser extend, Ice usmash and SH nair. ROB's anti-air options, besides just trying to outspace with DDs, are... SH fair? Much slower and doesn't linger as long as Samus's nair. SH upair? Slower even more, doesn't linger in front at all. His only grounded anti-air is maaaayybe up angled fsmash, which is slow to start, end, and only lingers for 2 frames.
Combos? ROB wins here for the most part. Grab is easier to land and throws are a little easier to follow up on. But it hardly matters because Samus just needs to get the opponent away from her or offstage so she can start setting up her godlike zoning game again.
ROB has good things too though, don't get me wrong. He just has too many things to take advantage of for him to be a top contender as a whole.
At least he can CG spacies 0-100 into an upsmash for a kill, regardless of DI. Too bad platforms get in the way.