This week's Your Nerd Side Show:
The Star Wars prequel trilogy built a strong case against the Jedi's capability to confront Sith lords head-on, the most profound through-line being isolation from their rival and inexperience. Although the Jedi institution was filled with powerful and skilled Masters and Knights, their combat flare and grace were no match for the unmitigated passion and rage behind a Sith's own technique. This was why even an apprentice like Darth Maul could take on both Qui-Gon Jinn and Obi-Wan and even kill one of them.
That being said, the Jedi were not completely incompetent, and their knowledge and abilities brought diplomacy and general peace to the galaxy for a thousand years before the prequels. Here's why that was as much their curse as it was their gift.
The Jedi Hadn't Confronted the Sith for a Thousand Years
Master Ki-Adi-Mundi said it in The Phantom Menace, "The Sith have been extinct for a millennium." So for a thousand years, at least, the Jedi had no contact with anyone powerful in the Sith arts. That lack of experience would be a grave detriment to any institution's understanding and training of future generations. Their knowledge was obsolete and outdated about the very people they were meant to protect the galaxy from. Lightsaber techniques would only be tested against the likes of other Jedi and enemies equipped with weapons inferior to a lightsaber.
When Qui-Gon Jinn and Obi-Wan had a great lightsaber fight against Darth Maul, they were still not enough to best him quickly, and only a stroke of guile from Obi-Wan got in Maul's way to victory -- but only after Darth Maul had already killed Qui-Gon. Throughout the prequel trilogy and even The Clone Wars animated series, very few Jedi, save for Obi-Wan, Anakin, and Ahsoka, could keep up with the Sith, and even then, just barely.
Sith Lords Were Not the Jedi's 'Speciality'
In Revenge of the Sith, Obi-Wan smugly, and ironically, stated to Chancellor Palpatine that "Sith lords are our specialty," just before a fight with Dooku where Obi-Wan was almost crushed by a metal platform. Furthermore, Anakin's first limb loss was to Dooku when he arrogantly dueled him in Attack of the Clones. Although it's fair enough to acknowledge that Anakin's hubris was the exception to the true Jedi way, it doesn't excuse the fact that several Jedi Knights fell to far-inferior users of the Sith arts because they failed to adapt their skills effectively through their training.
The most flagrant example of this failure was General Grievous, who wasn't a dark-side user but was trained in lightsaber techniques by Count Dooku. Although his dirty tactics and quad-arm-spinning actions made him formidable, his lack of resistance to the Force itself should have been enough for any trained Jedi to outwit or overcome. His incredible success, which led to a large lightsaber collection, was a stain on the Jedi institution's ability to adjust their training for the new realities of these powerful forces.
The Sith Surprised Even Jedi Master Yoda
Throughout The Clone Wars series, Jedi fell to those trained by the Sith, like Asajj Ventress and Grevious. Jedi also defected from the order from burnout in the front lines of the Clone Wars and changed sides like Master Pong Krell during the Battle of Umbara in Season 4 of The Clone Wars. By the time the Jedi institution was all but rubble near the end of Revenge of the Sith, even Yoda, the Jedi Order's most powerful Master, was defeated by Darth Sidious.
In effect, the Jedi were unable and largely unwilling to shift their techniques, priorities, and textbook ways quickly enough during the Clone Wars era to keep up with the growing threat of the Sith. Obi-Wan and Anakin held the strongest front during these times and were often considered renegades by the Jedi for their unorthodox ways. Even then, their victories soured under the rise of Vader from the ashes of Anakin's fear of losing Senator Amidala. The burnout of the strict Jedi way was very real under the threatening red glow of the Sith.