During Joe Kelly's run on JLA Plastic Man was shredded a few times when facing foes with incredible strength. His saving grace was that he could control every part of his body and the missing pieces could be absorbed.
So, about heroes like Plastic Man and Mr. Fantastic.
Any elastic material, no matter how strong, eventually has a point where it can rip or tear (at least to my knowledge). Meaning that technically, the weakness of heroes like Plastic Man is the same as the weakness of Silly Putty: stretch it too thin and it falls to pieces. Has there ever been a story the explored the implications of this?
Plastic Man actually has one of the more unlimited powers (Like Silver Age Superman and the like) because several stories have shown that there is no functional limit to the damage his body can take, including being cut up, stretched too far, etc. When writers get past his clown personality he's often used as one of the Grade A Power characters because the past has established that he essentially has no power limits.
Other characters, however, haven't been given such high limits and can be torn. Elongated Man, Mr. Fantastic and others have been shown to have a finite stretching ability and can be cut/pulled apart if the proper circumstances are met. Exactly how far that is depends on each individual writer, of course, and some take it way too far, but they are subject to the "stretch too far and come apart" consequences.
Life is like a roller coaster. It has its ups and downs, but if you sit back and relax you get a heck of a ride.
I think Reed is interesting, in that the physicality of his powers is very, very rarely shown or used. His genius is his real super power - stretching doesn't factor in much. Some issues have shown him being exceedingly in control of his pliability though, including stretching his eyeballs or reducing himself to a few molecules thin.
He seeks out Lex Luthor to find out how exactly his powers work and just how powerful he is. He doesn't go to Lex to become evil or a villain, he just knows Lex is the smartest man on earth. Lex captures Plas and along with the Scarecrow, they turn Eel into a paranoid timebomb with almost limitless power.
Plastic Man attacks the JLA and takes them out one by one. He's finally "talked off the ledge" so to speak by Booster Gold and Blue Beetle, both quite familiar with being thought of as jokes.
There HAVE been episodes where Reed has strained pr overstretched himself and has had to physically "reel in" the affected arm or leg. Kirby had it happen to him a time or two, as I recall.
Somewhat OT, but Tom's pitch made me think of it. I seem to recall an issue of one of the Marvel team-up books where Iceman had been brainwashed/controlled by Mesmero/whatever and Angel was trying to catch him. Someone asked him, basically, "What's the big deal...? The guy throws snowballs... He's a joke!", to which Angel replies, "No, he can STOP molecular motion just by THINKING about it and he is frickin' DANGEROUS!"
It's, as always, a matter of how far the immagination of the writer stretches. I remember reading one story where Doctor Doom captures the Fantastic Four (back then with the female thing, I believe) and puts them into death traps that are designed to find out the limits of their powers. Reed gets into a succession of ever smaller rooms, with the door to the previous room always closing behind him. In the end, however, he saves the day because in the very last, tiny room, there was a tiny crack in the wall, through which he escaped.
I think if you look at those characters as silly putty, then you will believe they have an upper limit. If you look at them as varying states of liquidity, then they don't. I always liked to think about them as a non-Newtonian fluid, which can be a fluid, or solid, depending on the force applied to it. Those characters can apply a force on a cellular level.