I can understand why people believe Ganondorf is a terrible character (lack of combos, speed, etc.) but i find this completely absurd. I have won every single game with him, and have beatin every character with him. I have even beatin classic mode, all-star mode, and subspace emissary all on the expert difficulty with him.
We do not include experience with the 1 player modes in the analysis of competitive play. Please lurk more.
I also would like to state that the only real problem i have found with Ganondorf is he runs a bit slow for someone who is supposed to be in shape (lol). I suggest if you are going to use Ganondorf that you should not rely on spead, but pure power. Smash attacks are his strong points along with his specials, so i would anticipate what the enemys next move will be. The best way to get use to him is to go 1 on 3 in V.S. mode with three fast characters (Sonic, Meta-Knight, etc.) thats how i got so good with him. I did the same strategy with him in Melee. I hope you will give Ganondorf another look.
No, his run speed is not his only problem. A lot of his hitboxes are terribly situational and he has massive blindspots. Furthermore, his ending lag is terrible on everything that actually has a good hitbox, meaning he will ALWAYS get punished by a patient player. His recovery is also easy to gimp.
His Smashes are all terrible. F-smash is probably his best Smash attack in terms of range, power, and general application, and it's SUPER SUPER slow and difficult to land, and massively punishable. U-smash has low ending lag, but a horrible hitbox. D-smash is massively punishable on whiff or block and doesn't kill anything. Furthermore, Ganondorf can do NOTHING to force people to think twice about shielding. His Flame Choke is easily spotdodged and punished on reaction, and his regular grabs have the worst range in the game. Mentioning spotdodges, Ganondorf can also do nothing against people who spotdodge on reaction. His fastest attack, Jab, has TWENTY FIVE FRAMES OF ENDING LAG, and 25 frames happens to be the exact duration of the average spotdodge, so in short, whiffing ANYTHING on a spotdodge means you will always get punished hard.
Ganondorf in MELEE is a completely different beast. In Melee, he has TONS of ridiculously long ranged stuff that has very low ending lag, not to mention an amazing recovery, and his throws actually combo extremely well. The advent of wavelanding means he also moves MUCH MUCH faster than a character of his type normally should.
Try actually playing someone who knows how to wait or poke with fast, safe, high priority tools. There is almost nothing Ganondorf can do in competitive play.