Samus is absolutely amazing in this game. Not top tier, but definitely high tier. Her main difficulty competitively is lack of representation and a complex neutral.
Personally, I feel like the difference between upper mid and high tier lies in matchup spread, and I'm not really confident enough in some of the more obscure matchups to feel comfortable saying that she is definitively high tier. I will say though that I feel like she has a lot of even matchups, including against good characters. For example, I think Samus vs Lucina is even, which is significant because that's a problem matchup for many characters. When I play Samus, I feel like I always have the tools to win if I play the matchup correctly, but it's rarely an easy win vs good players (ie, I don't think she has too many significantly advantageous matchups).
I dabbled casually in Samus starting from a little before the Melee -> Brawl transition and I have to say Ultimate's version of her feels leagues smoother than any prior entry. Like, in 4, I hated her nair and could never get it to work right. But in Ultimate I'm smacking faces with it regularly? I really don't understand what changed there. Startup frames I guess?
I feel like Samus just works in this game. She feels like a complete character in ways that she never did before, not even in Melee (there she's more of a gimmick character, though it's a pretty good gimmick). Like it's nice that her multihit moves actually link properly now, and Charge Shot feels appropriately powerful for the investment you put into building it. Being able to charge in the air is great too, and I feel like she benefits from the overall system changes more than most (most notably having a good disadvantage state is quite valuable in this game, though having access to her good tilts out of run is also nice).
Phan7om recently said that top representation is bigger than any buff or patch, and I agree with that 1000%.
I'm also definitely on board with this. Like come on E
SAM, Samus is finally a good character and you haven't even touched her in Ult? Where's the unending faith that you had in this character from 4?
One thing I'm never sure about on Samus is how much I should value fully charging Charge Shot vs just shooting it. Any insights on that?
It probably depends on the matchup. Against speedier characters, I always go for full charge and hold it as long as necessary, because you're not going to get that many opportunities to fire it, when you do get the chance you want it to hurt like hell. Against slower characters I fire more partially charged or uncharged shots.
A lot of characters require complex MU knowledge to use and operate, Samus / DS simply has fewer winning MU's when compared to high / top tier characters and overall in her own character archetype she is outdone by multiple other picks. That's her biggest problem, she's outshined in pretty much every department and it's easy to get stuck fishing for kills with charge shot reads if you push people into too high a % range.
I feel that Samus's biggest problem is what I call "honest character syndrome". Samus doesn't have some super braindead way to take stocks / close out games, you kinda just have to make the read or get the punish with charge shot or find that clutch back air or dtilt. I feel like she has the tools to win almost any matchup, but you're not going to get that read all the time, that's just the nature of the game. Granted, this has always been my style in Smash (In Smash 4, I played Yoshi, who IMO also works this way, except he uses DownB instead of Charge Shot or hits clutch up airs), but the very high tier characters are generally defined by more consistent ways to close out stocks, generally coming from safe, fast, or otherwise strong neutral options that lead into kills. I certainly wouldn't say she struggles to kill, but it's not trivial.