It's not every stock no, but being gimped is a huge problem for Lucas imo. The main reasons for this being how long PKT takes to initiate the hitbox, its ability to be easily rammed through and resulting in Lucas being spiked, and zair's extended hurtbox. Yes, Lucas can stall with magnet or strafe with his incredible air speed and his PKT2 goes much further than Ness', but at the end of the day, it's much more difficult for him to challenge the onstage opponent than the opponent to challenge Lucas. The worst trade the opponent can get from that scenario is probably bair or dair, both of which are finnicky and puny-hitboxed moves that require the opponent to seriously mess up to pose any kind of threat. He's definitely not Marth/Lucina/Shulk/Cloud/Mac tier in the recovery spectrum, but not far ahead of them either.
Also, there are tons of people who are probably much less credible than even myself. I'm pretty bad, maybe top 10 in my inactive and terrible state (which is very small in number compared to east/west coast powerhouses), but to say I'm not the most credible player to talk about his viability is kinda moot here.
I guess I should reword what I said. It's not his defensive ability that's bad, it's just... what does one get out of playing defensive as this character compared to someone who plays offensively? In my experience, faster characters can just dance around Lucas (not being 2-stocked, just using colorful language) and wait for him to commit to something punishable, essentially just out-waiting him and his bad frame data/hitboxes. Speaking of bad hitboxes, jesus christ have you looked at nair. It's one of his main combo tools, but can't even hardly catch and opponent running past him blindly without just auto-escaping it cause the hitboxes are TINY. Zair is good, but I feel like saying he has like 3 good moves and that doesn't make him bad is like me as a Brawl Ganondorf player saying Ganon had 3 good moves in Brawl and that was ok. Lucas has incredible close-up game no doubt, a frame-2 jab is nothing to bat an eye at, but frame data things don't allow for him to mix up that jab with anything to convert off of if the opponent is ready for it.
A frame-17 smash being a character's best smash is kind of a pathetic thing to defend, honestly. It has decent kill power, and it's definitely usable? But it's just not enough along with his other tools to keep him anywhere above low-tier imo.
As for him being overrated/not, pretty much every tier list I see places Lucas in high mid or so, and I don't think he has results that back that claim up. Jigglypuff, the character most people consider to be worst in the game, has more results than Lucas. I realize that Jiggs is MUCH more popular than Lucas amongst smash veterans, but I don't think there's anything so strong going for him that one could justify him being above mid tier. I dunno, maybe I don't pay attention enough to tournament results since I mostly focus on my own fundamentals and don't do much spectating, and I do not by any means think I am the most credible Lucas main--I've just thought of the things you mentioned before and disagree.