When you realize how mentally taxing and technical Mario is, it's really no wonder why we only have a few players who are able to get respectable results with him.
Despite being labeled as a beginners character by the Dojo, I'd say that Mario is in fact one of the hardest characters in this game to play correctly. He doesn't have anything extremely abuseable, so you constantly have to be alert in order to make the best use of his options. It sucks a lot when you whiff, and not paying attention or making a technical error makes it very easy to whiff with this character.
Pierce and I have analyzed Mario a lot in one conversation, and we both came to the conclusion that Mario is also extremely bait oriented. This in itself increases his learning curve since baiting requires a lot of skill to pull off correctly.
Anyhow, I was bored, and I did research on Jab combo frame advantages. The results were very depressing. It seems like there is like no Jab combo in this game that is safe between hits for the most part, since as long as you have a 6 frame shieldgrab, you'll at minimum trade hits by shieldgrabbing at the right time between hits.
Snake, Mario and Luigi's Jab combos for example have -6 advantages between hits, so when you shieldgrab correctly, you will take 2% and grab them.
Ike's Jab combo gets shieldgrabbed without trading hits if you time it correctly. Oh and by the way, Ike's shieldgrab is 7 frames, so he cannot shieldgrab between hits against -6 advantage Jab combos. Same goes for Ganondorf, Bowser, Donkey Kong, and other people with slow grabs.
Falco and Wolf's Jab combos are also shieldgrabbable between hits without trading. Against Falco's rapid Jabs, you will need an Up-B out of shield however. Fox's rapid Jabs on the other hand I read somewhere that you can shieldgrab between hits.
Against non-disjointed Jabs, you can grab them from pretty far away if you perfectly buffered the shieldgrab due to said character extending their hurtbox.