I didn't really buy into the Kazon sect thing and them basically stealing all of their technology from the Trabe and still being a threat to Voyager as the ship moved on whilst warring with themselves. If Voyager made a beeline for the Alpha Quadrant Maj Cullah shouldn't have even been in the picture after a few weeks. So i would agree that the Kazon were kind of a miss as villains. The Vidiians were cool though. They were setup as sort of a broken mirror of the Federation. A peaceful inquisitive people driven to commit terrible atrocities in the name of survival.
I actually like the Kazon and I also find their background story believable.
It has happened from time to time in history that a country with a people who has been liberated from an oppressor has turned into chaos when the resistance movement or the political opposition to the former dfictatorship or occupation power have split up in different factions and start fighting on another.
That happened in Iraq when saddam Hussein was overthrown. The US had anticipated that a democratic opposition should take over and make Iraq a democtaric country.
But there were no "democratic opposition" in Iraq. Instead there were several warlords who had fought Saddam's regime but now started to fight each other.
So the Kazon background story with rivaling sects is highly believable and actually worked fine as villains in the series.
They actually stole their technology from the Trabe and were obviously smart enough to make it working rather good.
And wasn't Culluh and Seska a lovely couple?
The only things which weren't so realistic were the water problem the Kazon-Ogla had on the Ocampa planet and the enormous width of the Kazon Empire.
If the Kazon-Ogla on the Ocampa planet needed water, they could robably go out and find some icy planet somewhere to get water, especially since the Kazon-Ogla main area seem to be located far from Ocampa, in the area where the moon Tarok was located in the episode
Initiations.
The most realistic explanation is that the Kazon-Ogla on the Ocampa planet was unde siege from some other Kazon sect, most likely the Nistrim and therefore couldn't get any water since the planets in the Ocampa system are burned-out rocks and a gas giant.
The kazon Empire also seem a bit too wide, considering the hostility and fighting between the sects. I mean, it took two years for Voyager to pass through it.
Note also that Voyager did seem to travel in some small corridor between Kazon and Vidiian Territory where smaler independent planets exists, like Sikaris, The 37's planet, Rakosa. Banea, Numiri, Ilidaria and others.
As for the Vidiians, they are interesting and good villains as well. However, a mystery that such a technologically developed species couldn't come up with a cure against the Phage or simply weren't extinct because they couldn't.