Coming from someone who didn't use (or fight, really) Corrin at all in Smash 4 and now practically mains him, he was very tastefully nerfed. They took away the things that gave him a bad reputation (pin in neutral, fair combos, stuff off of uncharged or medium DFS), while still keeping him a good juggler like he's meant to be, in a way that feels right at home in Ultimate.
I'm not really sure where I think he stands. He's a solid combo fighter and more than functional, but I don't think he has anything that makes him particularly "scary." Corrin can juggle you well, but that only racks up damage and exists separately from his kill options; and by that, I mean, he can't like, carry you to the top of the screen like Mario, string you into a nasty situation offstage like Inkling, or combo into a quick and powerful move like Dr. Mario. Regardless of what you think about those characters specifically, those attributes deserve respect. What I think is his scariest attribute is that he's amazingly good at covering any area in his disjoint range. His only dead zone is the distance where someone can foxtrot in and be up next to Corrin while avoiding his aerials. Or to put it more simply, just right out of fair range. He basically has a big bubble of "no u" around him and there's nothing like a diagonal or underneath weakness. (Well, being above your foe is almost universally a disadvantage, but Corrin has it better than a lot of other characters.) A bubble of "no u" that can combo you for 40% every time you step in has a lot of potential! It's worth mentioning, too, that he can kill super early with fsmash, tipper pin, and
especially DFS's max bite, those are surprise kill options and take a lot of work to be weaved in. I do think they'll become deadly and more important as time goes on since they're just that worth going for and Corrin's ability to control space can open up opportunities where the risk/reward is more in Corrin's favor.
Like you said, getting the kill can be difficult and it was one of the biggest hurdles for me when I started using Corrin (and still gets me at bad times in intense games). Staling is something you always need to keep in mind. Our only safe, spammable raw kill options are bair and uair, the former of which has some startup that makes it tricky to fit in and the latter of which is usually staled from combos/neutral. And this is probably just a flaw in my own gameplay, but I also find that the side B kick gets stale since the move itself has a lot of range and I use it to punish bad spacing a lot. It's sometimes my only punish option if I'm too far away. The kill setups he has are either limited (like falling nair/uair -> bair at ledge, which kills early, but only at mid-high percents and only at ledge), or require tight precision and/or timing (nair/fair -> DFS or pin, or fair -> fair -> uair, both of which also require golden percents). I only get maybe a quarter of my kills in the 90%-110% range. I either get the surprise kills I talked about and kill as low as 50%, or I have to slog away until I can up throw or up air them.
He also has trouble dealing with opponents' shields and getting out of his own. (Or really, anything that has him getting close to the opponent, since his startup isn't the best.) If you pressure his shield, he can only use his frame 6 nair OoS since all of his other options are 9+ frames and take too long to come out against most characters. (Uair is frame 7, but takes longer to cover all areas and hits too high for a lot of situations.) And if he tries to pressure your shield, he can only space his attacks (which do little shield damage and puts him in a position where his timing can be read), try to shield poke with fsmash or pin, or grab (which exist in his kit SOLELY to counter shields, or uthrow at high percents), and you can pretty happily stay in shield while he figures out what to do.
In addition to what I've said, here is a good analysis of the character from Twitter. I tried to talk about things that weren't said in here, since I agree with almost everything and I'd recommend reading it. And in the thread's comments, there are some Corrin labbers (or just people who frequently tweet about their thoughts using the character), so you can check them out too if you'd like to get a better understanding of this character.
Aaand lastly, feel free to correct me if I've said anything stupid about this character. ^-^
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