Lucario can down b cancel super and both have a hitbox and be invincible. If you are in the air you cannot clash with this, and since he's invincible, you can't hit him either, so basically if you are in the air and he gets to the invincibility frames of his down b there is nothing you can do. I'm not saying he can do this on reaction from any distance or anything, but it does affect how close you HAVE to be to him in order for him to not be able to react if you want to approach from the air.
Also being able to punish a character when you predict a jump (regardless of their fadeback or would-be safe aerial placement), is not insignificant. Sure, it costs a charge, but it also only loses to spot dodge/roll air dodge, and even then the lucario can do fsmash / reverse fsmash out of the down b to cover some of those options.
Plus, Lucario's regular non-canceled down b presents similar problems simply because of the threat of a charge use (assuming he has a charge). After all, in order to not get hit you have to shield or dodge, (and in order to avoid shield pressure you have to dodge which is more committal). This means that while you COULD grab him on reaction during the endlag if he down b'ed without a charge, this is pretty risky as long as he has a charge. Also, while you could attempt to shield on reaction -> jump w/o reacting out of shield after predicted possible hit -> aerial his non super-ed down b endlag, he can ASDI down tech most moves at lower / mid percents and hit you for it. Not to mention the fact that you can't consistently predict when he'll hit shield cause he mix up when he cancels it and mix up what moves (which have different start up) he uses out of it. Edit: and if you try to wait and react to the lack cancel, you can't hit him in time unless he ends right next to you (which is pretty easy to get if you don't shield/dodge but up to his spacing if you do).
Then there's the fact that, super-ed or otherwise, if he down b's at you and you choose defense (which is needed to deal with the super-ed version) he can switch stage positions with you.
Now I'm not saying Lucario's neutral is god tier... but it's certainly not bad. Especially coupled with an on hit cancelable disjointed dash attack. Imo having dash attack be a solid straight up approach is questionable because it's available to him in his dash state, which is normally an effective state for positioning at the cost of options.
One thing I will say that I think DOES need a change:
Lucario's aerial down b super is one if not the worst designed thing in PM currently. You can't clash with his hitboxes cause he's in the air. He is simultaneously invincible and has a hitbox out (you do not lose invincibility before the hitbox comes out). It's kinda a dive kick with huge range (meaning you can use it to get to the ground which is nice against juggles). It's sorta angle-able if you include the down air shenanigans. So like... you can pretend to try to edge guard him to get him to waste a super (and hopefully you don't get hit), or you can wait to punish his landing except since you're not near him he has no incentive to use it. Basically if Lucario is high and has a charge, he's coming back, and if you try to stop him you'll take a charge from him, but you are also giving up center stage.
tldr: I agree with Lunchables that Lucario is top tier.