Well, I guess it depends of the matchup, the skill of the players involved etc.
In brawl, I played mostly against a better player than me, who mains MK and Marth. Against those characters, it's basically a suicide to follow them offstage with zelda (safe upB, will most likely send you under the stage). Even against other characters, I believe that it's really risky : a single random hit can end your stock if it sends you in a bad spot for up B (then or you can't make it back because you are under the stage, or the guy grab the ledge before you do so and you are ****ed). Moreover, if you fail, the guy will most likely come back on stage before you (because the up B in Brawl is really slow), and I guess we agree to say that she is really easy to edge guard in Brawl.
Let's say you won't get hit and you won't fail, you still have to be carefull to not go by yourself somewhere you will have troubles to recover from, and your opponant knows that, so it's easy for him to anticipate your movements.
On the other hand in sm4sh, as you said it's easier to come back on stage. You can basically go where you want, you will still be able to upB safely and grab the edge (and he can't steal it from you). It allows you to be less predicatble with your offstage game, and you can get hit without dying.
About the DownB, my way to use it is to create a wall in front of the edge (I use it from the stage, and I don't try to hit the guy with it, I just want the soldier to be there) so the guy can't go through it. This way he is easier to spike (because it limits his options). You can also use it against some UpB (still from the stage), when they try to come back from the low/side, they can't go through it so they can't grab the ledge (works really well against link, shulk etc.).
In a game where it's hard to end a stock early, if you have one of the safest upB and one of the best spike, you are really scary offstage imo : one spike can win the game.