On the one hand, he had a prominent role and access in making major policy decisions it seems. But since Nixon did not pardon him, he turned on Nixon hard. Given his privileged access to how top-level policies are formulated, should he be a source that is suspect because of his well-publicized dislike of Nixon? Are the posthumous remarks credited to him an honest discussion of the Nixon administration's policies? I've seen his remarks repeated as if it is now in general accepted fact for some people. I also have seen it criticized as well as untrustworthy.
When approaching any source the first thing a historian must do is consider the biases of the source. Why was the source created? By who? What did someone have to gain? Was this source created at the time of the event? A day later? A year? Was the person who created it there for the event, or did they hear about it second hand? All of these factors and more need to be assessed when deciding how to use a source.
In the case of any post-scandal testimonies about a public figure, it is important to keep in mind the narrative that the testifier is building, as well as how that narrative fits whatever hard evidence can be obtained. In the case of Richard Nixon, the best source for hard evidence about his views are his actions and the Oval Office Tapes (which can be found here http://nixontapes.org/transcripts.html). I'm going to note here that these tapes are often a difficult thing to listen to, as they are often quite vile in their content.
A good litmus test for whether the actions of Richard Nixon had ulterior motives is to see what he says on the Oval Office tapes, which seem to the listener to often have little filter. In terms of the idea that the drug policy was an attack on is opponents, I have never heard him say as much on the tapes. That being said, he has little good to say about American Jews, African Americans, Irish, native Americans, Italians, hippies, liberals, etc. Therefore, a historian might conclude based on the Nixon tapes that Ehrlichman is a trustworthy source in this regard.
On the other hand, a parallel can be drawn with Nixon's predicesor Lyndon B. Johnson who also had tape recorders running at all times in the Oval Office. Here we find racist, sexist, homophobic and otherwise vile remarks along the same tact as Nixons (information about which can be found at https://millercenter.org/the-presidency/secret-white-house-tapes/about-johnsons-secret-white-house-tapes). As with the Nixon tapes, I'm going to warm that these are a hard listen. It is often difficult to rectify the pro-Civil Rights presidency of Johnson with the hateful views he espouces in some of these tapes.
The point of all of this is that history is messy. Sources are almost always difficult to interperet. Having more sources often just makes the other sources less clear, but is necessary to get closer to what happened. As I always emphasize to students: do your own thinking, hear things with your own ears, see them with your own eyes and keep in mind that any "history" is just one person's opinion of what happened, with the strength of the argument being depended on the use of the sources.