Some dash grabs can indeed be better, and boost grabbing can also be better than a JC grab. By better though, I only mean for grabbing with more range or with a better boost in momentum. JC grab sometimes will not take you far enough out of a run or dash, or a character will have an amazing dash grab like Ness. When it comes to endlag, usually you will experience much more lag for whiffing a dash grab than a JC grab, so the trade off for those characters is a "better" effective range with harder punishes if you miss. On the other hand, some characters have very crappy dash grabs because they give little "boost" forward when used. In those situations where a dash grab has equal or less effective range, it's always better to JC grab out of a dash because you will retain similar or better momentum AND have the benefits of a less laggy grab
On a more obscure note, dash grabs sometimes have noticeably different grab hitboxes or locations. A perfect example of this is for grabbing short or crouching characters. A standing or JC grab might miss these characters by grabbing higher, where as a dash grab could probably scoop down lower. The obvious reasoning being that out of a dash, your character is already in motion and would probably not grab with 50/50 perfect standing balance, so you would be leaning down a bit or would be slightly less tall from leaning forward. Animation wise you can probably notice a few dash grabs on taller characters seem to have the character crouched slightly or grabbing from less than full height.