So, we all know that Ulic-Qel-Droma was renowned for his knowledge regarding the immunity or resistance against the Dark Reaper that was lost with time.

Now, a few of us have assumed that the Dark Reaper feeds on Force energy like Sith Lords: Darth Nihilus and Lord Vitiate and that renders Anakin Skywalker/Darth Vader immune/resistant to the drain of Nihilus; this unfortunately it is not true.

Now, the Dark Reaper has inside it a device called the Force Harvester.
A lot of people assumed that the 'Force' word refers to Force energy; this is not true. In fact it was already established that the Dark Reaper needs Living Force to function(stated by Mace Windu). The Living Force represents the lifeforce; whereas the Cosmic Force represents the Force energy.

"You should be honored. You will be the first Jedi in a thousand years to feel the effects of the Harvester. You lifeforce will be absorbed , as well as everything else in this forest, and in the end, I will destroy the Republic you fought so hard to defend."
―Count Dooku to Anakin Skywalker

In the Revan novel it is clearly mentioned a difference between the two 'forces':

"The ritual had obviously destroyed Nathema, snuffing out all life on the world. Lord Vitiate had offered his people hope and instead had brought them a fate worse than death—utter eradication of life, existence, and even the Force(...)."

―Revan

Now, let's talk about the Dark Reaper immunity

"I can teach to harness the power of the Force around you, making you immune to the Dark Reaper's effect, for a short time. But this knowledge comes with a risk." ―Ulic Qel-Droma to Anakin Skywalker

Now, if you mark Ulic's words; he also says that this knowledge comes with a risk. But why? Well, that's because the immunity to the Dark Reaper comes with the technique of absorbing lifeforce around you.

"(...)to harness the power of the Force."

"(...)knowledge comes with a risk."

Also, Ulic says it is: "(...)for a short time."

Why? Well we know that the Dark Reaper, once activated, it unleashes lightning bolts of Life-Drain. And the more it consumes; its range the greater becomes.

Thus, in order for Anakin to resist the Dark Reaper's effects; he has to drain the lifeforce around him, hence: "(...)for a short time." and "(...)knowledge comes with a risk."

Below you have a depiction of how the Dark Reaper works
00:31

https://youtu.be/HYEc8C_1gTc