Playing devil's advocate for a second, we only really get to see a handful of scenes in the movies where the Rebels are battling Stormtroopers in armed combat and, frankly, they seemed to perform pretty well in most of them though in none of those scenes did we see the hordes of them that we're used to seeing in the Clone Wars series. Let's look at the record for a minute:
They mercilessly overwhelmed and killed the crew of the Tantive IV at the beginning of ANH, destroyed a Jawa Sandcrawler (off-screen), murdered Uncle Owen and Aunt Beru (off-screen), destroyed the shield generator and killed lots of Rebel pilots and ground troops on Hoth while piloting Walkers (ESB), they successfully stormed Echo Base once the shield generator had been destroyed, they helped capture Han, Leia, Chewbacca, et. al along with Vader and Fett on Bespin, and, of course, Imperial troops apprehended (albeit briefly) Han, Leia, and the rest of their strike force on Endor in ROTJ.
Of course, Han, Leia, Luke, et. al did manage to evade the stormtroopers at Mos Eisley and at the Death Star but had to use a lot of tricks and other methods (sneaking into the detention area, swinging across chasms, hiding behind blast doors) to successfully evade them. There is also some suggestion made by Steve Perry's "Death Star" novel that the Empire was "compromised" by some traitors in their midst that enabled Han, Luke, Leia, et. al to escape. The Rebels did manage to escape from them on Cloud City but they did have some assistance from Lando's security officers in their escape. Plus, they were only dealing with a limited number of Stormtroopers (fighting in an unfamiliar location BTW). As far as ROTJ is concerned, the Imperial troops WERE eventually defeated but it was only through the combined efforts of the Rebels AND the Ewoks. It wasn't like the Ewoks defeated them singlehandedly and the Empire didn't/wasn't able to provide any backup for their limited number of troops there. Had they not been occupied fighting the Rebels in space, the Star Destroyers could have simply burned the planet to a crisp like they did several times to Kashykk and/or dispatched more troops to overwhelm the Rebels and Ewoks at the shield generator. Furthermore, the Emperor was convinced that he would win no matter what and was more preoccupied with the unfolding melodrama between himself, Vader, and Luke.
I guess we'll never know for sure whether or not the PT-era clones would've performed better/differently than OT-era soldiers but, simply based on what we see or know of in regards to the Stormtroopers it's not clear to me that they're all just a bunch of incompetent buffoons.