it depends on what character and what DI. for some real fast fallers like fox it'll work even with DI, but for more floaty characters it only works if they just go straight up.
and yeah it does have a limit, about 150% depending on which character, but i feel the KBG is quite low, so there is quite a big range of values for this to work. it does work on falcon at about 150% (just cause hes so heavy, anything less than that wont kill him anyway)
then again, fox's upsmash does kill at 100%+ lol, but still a useful tool. especially in fox dittos
oh and u forget cowtipper, fox is a speed demon through the air, he is just so dam fast lol
does the upthrow chaingrab fox btw?