«Modern» Sith is a very poor choice of words here. There are (as far as I know) about four different distinct Sith traditions throughout the entire Legends continuity:

• The Ancient Sith (or True Sith) who inhabited Korriban, including Sith Lords such as Marka Ragnos. Believed to be extinct at the time of KOTOR, they were eventually revealed to have survived in secret, and openly emerged to challenge the Republic in the Great Galactic War.
• Assorted «Cargo Cult Sith», such as Exar Kun, Darth Revan, and the antagonists in KOTOR II. These were fallen Jedi who adopted Sith trappings and teachings. They were more or less wiped out by the re-emergence of the Ancient Sith.
• The New Sith, who united remnants of the Ancient Sith with Cargo Cult Sith from the Jedi Order. This is the Sith tradition that Darth Bane originally belonged to, and which was conclusively defeated by the Jedi a thousand years BBY.
• The Banian Sith, including all the movie villains.

The Ancient Sith possessed a much deeper understanding of the Dark Side than the Cargo Cult Sith did, because their civilization was dedicated to studying it, whereas the Dark Jedi were mostly just picking through the ashes. It’s the Cargo Cult Sith that Kreia is referring when she speaks of «modern» Sith.

The New Sith are tricky to rate in terms of power relative to the previous groups.

The Banian Sith were by far the most powerful Sith organization in the galaxy’s history, eventually resulting in Darth Sidious, who is canonically the most powerful Sith Lord ever in both Canon and Legends.