The standard test is, “if both characters hit their fastest button or perform their fastest safe special within range upon recovering from the move, who wins?”
Often +1 does the job, especially at point blank when both characters have access to light buttons with 4 frames of startup (and mediums with 5 frames of startup). But depending on the characters and the spacing, +1 might be insufficient, or even unnecessary. Also, extra frames of advantage make it more difficult for your opponent to test your sloppy execution by attempting to press a button, allow you to catch your opponent in pre-jump frames if they try to jump for a grounded combo, allow you to catch your opponent at all if they jump and they’re further out and you have to press a slower button to check them, etc.
In your example, you wouldn’t get a frame trap in a technical sense, no, but you have access to other light manuals which do give Shago advantage, and I doubt anyone is noticing that you don’t have frame advantage and pressing a button if you go for a reset after LK anyway.
As for your previous comment,
A crossup attack, particularly an unreactable, barely reactable, or ambiguous crossup, is a mixup. Besides which, Shago has access to a slide and a fast overhead command normal (back+HP), so I think he does just fine on the high-low mixups (as well as, in fact at the same time as, the left-right stuff).