I've been playing Dedede a lot the last month or so, sometimes I pull him out at tournaments because he can beat players inexperienced in the MU pretty reliably. But I think he's bad and I don't understand why Japan has him ranked so high.
He loses to so many playstyles it's unreal, zoning in particular, even by characters that aren't great zoners, just totally craps on him, sword fighters generally beat him, and the rush down as well. He has a terrible matchup spread and he does not secretly beat any top/high tiers at all.
Huge hurtbox plus almost non existent air speed plus slow on the ground is a terrible combination, but for whatever reason his combination of weight, fall speed, and gravity is absolutely perfect for making him get put into tech chases all the dang time, even by random moves that don't put hardly anyone else in the cast into a tech chase situation. He may very well be the worst at CQC in the game, not only is his jab frame 10 but it doesn't link properly on many characters. His dtilt might keep it from the worst in the game, but regardless, it's terrible.
He has a big grab with big reward. Gordo is pretty good, especially for ledge traps, his neutral is moderately good with his aerials, ftilt, I guess his command grab lol, and his grab, but it's easily sub par and ruined by any amount of zoning from the opponent. And his stinking frame 6 jumpsquat and 6 frame hard landing lag, he can't wall as effectively with aerials as he could otherwise.
And he can't kill very well. I can't wrap my head around why Dedede was designed with so much trouble killing, what exactly were the developers going for? It's not unusual to have to wait until 160-180 to get kills with Dedede, unless you land a stray uair, or dsmash with a good read, or bair with a really good read/mistake by the opponent.
I've played Samus vs Dedede from both perspectives and it is atrocious for him. Samus' ability to force tech chases and Dedede's inability to avoid them leads to CS over and over, any hit confirm leading to 40+ damage easily with his size, zoning with zair made exponentially easier by his size, Samus' frame data wrecking him especially CQC, Dedede's trouble killing Samus but Samus getting kills much more easily off CS tech chases, there is just so much wrong with this MU. It's a random 30:70 MU Dedede really shouldn't have.
He has strengths, yeah... He has a really good grab game (wish it could kill), strong disjoints, his recovery is great, 5 jumps... But his weaknesses are huge.
As for the whole, "Buffing him is dangerous thing," no it isn't at all. I think it would be ridiculously challenging to make Dedede an actual dangerous or overpowered character lol. People used to always tell Samus mains that she didn't need buffs that were too big because it was a, "slippery slope," whatever. Samus got huge buffs and she isn't exactly dominating the meta. Bowser and DK got kill confirms, they don't dominate, Dedede absolutely deserves big buffs and I really want to see them.