pfffft speed and shield are usually the arts most worth using
Being stuck in those is usually a benefit
Jump isn't that bad because you can outcamp or chase those who try to do camp
I wouldn't use buster or smash with normal Shulk anyways unless I had a really great opportunity
Risk/reward isn't awesome when defensive options are great this game
Buster is actually good (if you can hit, and usually I do) because of the damage output. Hyper Buster takes that up to eleven. Just landing one hit in hyper buster deals enough damage that you can cancel. Shield damage is also amplified. If you're locked into it though, you lose all the benefits. Sure you deal slightly more damage, but then you get hit. Want to cancel to avoid taking more damage? Too bad.
Smash sucks though, and should never be used unless:
- you have rage
- you have 2 stocks left
- your opponent is on their last stock and is at kill percent.
Hyper Smash is slightly better because it doesn't last as long and you can kill most of the cast near the ledge at around 80%, but if you get hit, you're dead. Hyper Smash is easier to kill than Jigglypuff. It's a gamble that I hardly ever use.
In regards to risk, when you select anything in Decisive, you're putting yourself at risk for an extended duration. Jump means you take more damage even if you combo slightly better. Your grab still sucks though. Speed is probably the least risky, but it is important to note that you deal less damage. In most matchups though, you'll want to be stuck in speed, so I'll give you that. Shield is good if your opponent doesn't know how to beat it. If you're locked into shield and your opponent knows what they're doing, you're going to die due to Shulk's poor recovery, the inability to switch to jump, and the ease of combing shield Shulk. If you pick Decisive Buster or Smash (especially the later), god help you. The risk is not necessarily removed with default or hyper (Hyper Arts amplify the debuffs after all) but they are more flexible because you can cancel. It gives less time for the opponent to adjust to the mode and figure out a counter.
I mean, picking Shulk is in an of itself, a risk. You'll come across Sheik sooner or later. I like being risky though.
tl;dr version: Decisive Arts make speed and shield (assuming your opponent sucks at edgeguarding) good, but everything else suffers due to the inability to cancel arts.
But hey, I'm not that good anyway. Decisive Arts just don't work for my playstyle. Customs are banned in my local scene now, so I'm going to have to use default regardless. (Yay, Smash is even more useless again)