It's a good out of shield option for pressure, like what X-Rey said. It used to have invincibility on the early frames, stuffing out a lot of burst movement options such as Sonic's spindashing, Yoshi's Egg rolling, Mewtwo's teleport shenanigans, etc.
Even though I use it less as a "Get off me" tool in this game (especially since you can't get the free charge at the start of the stock, which would fool a lot of players into approaching, keep in mind, that this move actually KILLS). I find the move pretty useless against Spacies, since their shine comes out before your Burst. So never really use this move against spacies when being defensive.
Seriously, having trouble getting a grab on a floaty for that up throw kill? Maybe they're at a too high percentage so when you down air them, they're much too high for your up smash, and you're not confident that you can get the back air perfectly spaced in time for the sweetspot? Welp, it's not a big deal, because you can double jump up to them while they're in hitstun (because the hitstun for Lucas's dair is very good) and then hit them with PK Burst. This has been something I've been doing for a while since 3.5 dropped, and I'm glad that Burst doesn't have set knockback (which I originally had no problem with the concept back when talking about 3.5 changes when 3.02 was a thing).
tl;dr if you don't like it as a way to combat pressure, use it as a kill move at high percent, and maybe even a combo/string extender at low percents. It's especially good on the floatier characters in the game.