Samus is fun, but she's definitely low tier. Lets get out the obvious reasons as to why.
Really bad neutrals. Homing Missiles are 100% worthless, Super Missiles clank with everything, bombs offer no stage control and leave you completely open, and oh god her grab. Her good neutrals are F-tilt, N-air, Z-air, and Dash Attack. N-air I'm iffy over but I love the rest of them.
She sucks in disadvantaged state due to be juggled and she has no real way to get anyone off her except for N-air (which is no longer a sex kick) and her advantaged state isn't worth a lot unless the opponent is off stage. She has combos and nice damage per hit, but without the neutrals to allow her to get into that state reliably you don't see much of it.
Dash Attack is her only real hit confirm move, and this is where she can go combo city on everyone. Falling U-air is her second best hit confirm move since the hitbox is so big and it can automatically lead into another aerial or Up-B. D-throw does the same but it's from her terrible grab, and D-air can get some stuff done but it's techable (ugh), and you kinda can get Falling N-air > Dash Attack.
Samus is really, really reliant on stage control. She cannot handle pressure at all. She needs time to charge her shot, she needs space to do an effective dash attack, she needs to be able to space with F-tilt. Thanks to missiles sucking it's just so much harder to maintain stage control because her lackluster tools are unable to make her gameplan work.
D-tilt and U-tilt are kinda just there. I enjoy D-tilt as a move and I mainly use it to punish rolls and get extra damage in, but overall it's underwhelming. U-tilt is only usable as a spike from the ledge, though you can get U-tilt > B-air as a kill confirm if your opponent doesn't tech (D-air is better for this though).
Why does U-Smash exist?
So we talked about the bad but lets talk about the good.
Samus' edgeguarding is legit. Z-air and F-air walls out horizontal recoveries, D-air is a quick spike, and Samus can tether ledge-trump to get guaranteed B-air kills. Bombs can be a safe way to gimp recoveries from directly below, and Charged Shot is always a safe way to edgeguard and force your opponent to go low.
Samus' kill power is actually very strong. She has one of the strongest B-airs in the game, Charged Shot gets work done, Up-B kills from platforms (kinda), and if all else fails she has one of the fastest F-Smashes in the game with a potent sweetspot. Great for punishing poorly made options.
Having trouble killing though? Just use F-tilt > Charged Shot. It gives a tech chase if they tech, and it's guaranteed if they fail to tech.
And lets talk about Charged Shot in general. Above and beyond, Samus' best move, and is the main thing that makes opponents really scared because they don't want to eat 23% damage. This move can easily shift momentum depending on how you use it. You can either throw it out willy nilly and force your opponent to shield in prediction, or you can hold onto it and surprise your opponent with a kill shot. Honestly, Samus' gameplan revolves around the mindgames of this move, and every high level Samus player uses this effectively.
That said her positives are outweighed by her negatives. I feel Samus would be more legit if her jab actually linked properly and her missiles could be fired at a faster rate. Unfortunately we have Sakurai-balanced Samus.
To note though, customs really, really help her. Relentless Missiles are amazing, and they kill confirm into Charged Shot. Megabomb is also powerful as stage control and I honestly have no idea why none of the Samus mains like that when the other bombs are so bad, and Apex Screw Attack has strong kill power even though I prefer Default Screw Attack for the more effect OOS option.
So yeah that's my spiel on Samus.