You, nor anyone else knows how unreliable the other FS moves may be.
You don't know how they work, how they intiate.
Could be as simple as, while your flashing you hit the other person with melee and your final smash sequence starts, instant damage/KO.
Mario Finale - Basically a Mario Kamehameha. Shoots one way in a cone-like form. I'm sure it can be avoided by jumping over it, getting below it, shielding/dodging, or jumping between the fireballs if it cones out far in a big stage. Probably the weakest of the 4 final smashes released, but depending on the stage size it might be the hardest to avoid, next to Giga Bowser.
Triforce Slash - Seems to affect only a single person for a lot of damage. Just get out of Link's way. The only way I can see this being even more dangerous is if Link dashes at someone and literally homes in on the person so evading is pointless. Probably the 3rd most dangerous of the 4 released final smashes since while it might work on one person, it'll probably set them up for an instant KO, if not already KO them.
Volt Tackle - Described to be a superpowered Quick Attack as it looks like. Shielding would seems to protect against it and since Pikachu is all over the place attacking everyone (if it's a small stage) then running is pointless. Chances are you'll get hit, but not with a huge amount of damage like the Triforce Slash or Mario Finale. You can also be near an edge so a Pikachu might not come after you in fear it'll kill itself if it wears off while attacking you on the edge. Overall, it's probably the 2nd most deadly of the 4 released final smashes due to it's unpredictability.
Giga Bowser - I say this is the most threatening final smash because it just is. First off, you have 100% full control. Giga Bowser goes where you say he goes. He does what you say he does. Also, there's very little you can try to avoid it:
-Shielding? Grab him.
-Dodging? Attack after dodge.
-Running? Hunt the ****er down and make him regret it, and also, on a small enough stage, running is pointless. Giga Bowser on Battlefield = Giga Bowser taking up 1/2 the whole stage.
-Attack to slow him down? Practically invincible so no point.
It seems the only way to protect yourself against Giga Bowser is to run away and hope he doesn't chase you. Considering all his moves will be all of Bowser's moves with 2x the power and range, and considering Bowser will probably have good attack priority already, Giga Bowser is a lot more dangerous than the other final smashes shown so far. Being in full control is more threatening than having little control.
This is all just logical speculation, so I don't want to hear a "you don't know for sure" speech from you again. I don't know for sure, but I can rationalize their effectiveness based on the information given.