So the effectiveness as PK fire as a tool is related quite heavily to the run speed of the opponent we're playing against. Or at least this is the impression I get. Pivot pk fire is a generally safe move, , but the best way for most people to punish it is to run up and shield it. Characters that are particularly fast are very good at punishing pk fire. PK fire gets punished very easily by Sonic, Falcon, Fox, Greninja, Sheik, Mac, and ZSS. Obviously we can still use pk fire against them, but it requires more thought and care than usual. I feel that other characters' run speeds are not extreme enough to diminish pk fire's effectiveness too much however.
Ness is 35th fastest dash in the game, along with Falco and Olimar. He's quite slow actually it's a bit shocking. However I think that all of the characters that are slower than him are so much worse at handling pk fire.
This thread has everyone's running speed:
http://smashboards.com/threads/smash-4-running-walking-speed-rankings.371564/
I would say anyone slower than Ness has a lot of trouble dealing with pk fire. Exceptions being Peach, Link, Megaman, Jiggs, and maybe Zelda. Peach and Jiggs approach aerially, Link and Megaman force us to approach because of projectiles, and I haven't played much against Zelda but perhaps Nayru's love is an effective tool against pk fire.
All of those other characters though are severely hampered by pk fire I feel. Ganon and DDD in particular have an awful time with it. It's very difficult for them to deal with pivot pk fire and dash grab mixups.
Meh all I'm saying is that the safety and risk of pk fire is affected heavily by the dash speed of the opponent we're facing. Extremely fast and extremely slow characters are very notable, and should be taken advantage of.