Monsters: A Sign of Narrative Strength.
I am a firm believer that the a hero can be judge by the richness of their rouges gallery. Many characters can work with relatively weak rogues gallery if their emotional demons are strong enough to torture them.
Charmed, honestly, has neither.
None of the monsters really prey on any well established fears outside superficial "otherness" and "Adversarial" tropes. The evil of the show is about as deep as an 80's cartoon villain—evil for the sake of evil and for the cause of evil. That can work, but for Charmed, it only served to damn it further in the annals of easily forgettable television.
The monsters generally do not stand out, for all the fireballs, celebrity guest stars and cleavage. Fans are hard pressed to think of a villain beyond "The Source" (or "All Evil"...which seems to change hands every season or so. Other similar Big Bads
followed upon his rather unceremonious defeat (and re-defeat when new demons mentioned below tried to take that power for another 2 seasons)), Cole Turner (a Dating Catwoman
story arc), Barbas and two demonic seers. If any others are remembered, it is only for the actor who portrayed them rather than anything deeper (like the fear they embody or tap into).
As the series went on, the storytelling weakness of the monsters became more apparent as they lost their otherness by the 4th or 5th season and simply became humans with powers that were bad and thus called "Demons" with few traits to mark them visually in their distinction. As someone with an interest in visual effects, this was rather annoying (not that the original designs were anything to write home about, often amounting to little more than face-paint and teeth).
Other weaknesses are well documented, but the monsters was the main thing for me. Its tendency for the writers to take "Inspiration" from recently popular supernatural films or franchises (Harry Potter style Witch-Schools, Adrian Paul playing out the plot for The Mummy
Remake, etc) only further damns it.
People compare it to Buffy
rather frequently, but in truth, it should be compared to Power Rangers
in terms of nonsensical silliness. But with that series monsters, they are unique and highly memorable (in most series), elevating its overall quality and allowing us to accept the cheese and Narm Charm
far more easily.