If I were to take a stab at it, I'd say that hypothetically FSN Lancer could kill a Goku or Vegeta simply because "getting stabbed in the heart with a spear that prevents you from healing/regen the wound" genuinely wins you a
lot
of fights.
Conversely, most DBZ characters probably could speedblitz and vaporize FSN Lancer before he can activate it (again, hypothetically, I doubt most would behave this way) - and vaporizing is key here, if they "just" punch a hole through his chest he could probably still pull it off with Battle Continuation and win-by-dying-last and/or draw.
So I think it really comes down to how the DBZ characters try to kill him. Because they can get a nasty surprise if they aren't thorough enough with it.
May you find your book in this place.
Formerly known as xp1337.