Unsafe when shielded, but it's not a bad tactic if you use it right, really. The amount of times I've combo'd off a fast-falled F-air landbox mixup is uncountable. It's great for catching opponents offguard when they try to punish F-air OoS, or if it whiffs (-or if you make it whiff on purpose. Dat bait.).
The norm hit of F-air unreliably combos on grounded opponents at lower %s into the landbox hit, though, if you hit the first hit, just carry on following up as usual, as it's not worth trying to FF into it unless you can 100% guarantee a hit, like on characters with larger, heavier hitboxes where you can set it up with D-air's landbox and anticipate their landing and fast fall to meet it.
(Tested D-air FF to F-air FF on Bowser, Fox and Ganon. Hits them all, though Fox is too small and falls too fast at 0% to combo from D-air. Starts working at 9%, interestingly enough. Probably needs more testing, but can't be bothered. You get the idea. :V)
I will say this though - neither smashes combo reliably if at all against most of the cast, and you can't seem to jump fast enough for any airs to either, so your best bet is either U-tilt or D-tilt, since both combo from the hit and have decent followups. F-tilt is pretty acceptable too, but, yeah, the other two will rack you up more damage.
This is nothing too new though in fairness, but any awareness to this little gem is never unappreciated. It boosts Bowjow's pressure and combo games quite a bit when you get used to his 2 landboxes and learn to use them. They're not gamechanging, really, but anything that can push that combo to have a few more hits is welcome in my book! :D