Short Answer A key point to realize in thinking thru your question, is that all empiricism is first person, and subjective. Science tries to harden its data as much as it can, by trying to cross check the subjectivity, making things as intersubjective as possible. But the closest we can ever get to "objective" is "intersubjective consensus".
Additionally, there are subject areas where one simply cannot go intersubjective, and empiricism is solely subjective. This is the case for almost all informal empiricism -- such as learning how to walk, or developing the technique to consistently throw and catch a boomerang. Many aspects of selfhood and psychology have to be purely subjective as well.
We do not have a settled ontology for the universe. For physicalist worldviews, none of these practices could possibly lead to truth as they presume non-physicalist ontologies. But since basic ontology is in dispute, that is not a valid prohibition.
For those worldviews which postulate a mystic/spiritual realm -- such as interactive spiritual dualism, or consciousness-based idealisms -- direct mystical experiences are in many cases the best, or often the only, way to access significant aspects of our world, and gain understanding of it. So, based on the critical role of first person empiricism in knowledge generation, and the possible case where the only source of knowledge for major aspects of our world would be mystic experiences -- the answer to your question basically has to be a "yes".
However (caveats) the accuracy of mystic communication is not high. The best documented accuracy data I know of is from remote viewing.
Remote viewing passed multiple statistically significant test cases in lab settings. In many cases, the process was to pick four very visually dramatic and distinct locations, and take photos of them. Then send a "target" individual out to one, and have the RVer try to sketch what was around the target. Then a separate judge would evaluate which of the four photos, the sketches was the best match to. If it was the location the target went to, that was a hit, if not a miss. Hits were statistically above the 25% random level, indicating some degree of psi-based knowledge. The CIA then tried a few location based studies, and got occasionally breakthrough levels of knowledge by this method. The hits were good enough to set up a program for 25 years. In the CIA program, multiple RVers gained insights on the same target, and the PM compiled these results into a report.
The RV PM summarized the accuracy rate of these cold location searches at approximately 30%. This was the accuracy rate that 25 years of iterations/variations on method, PLUS the use of as much intersubjectivity as possible, could get to. So -- any given claim or observation had over twice the odds of being wrong as right. Now for a blind set of guesses, they would fall far below 30% accuracy, as there are so many different possibilities that a real site could consist of,. so, just like with the lab data, the field application of RV was helping discover "truth". BUT -- with a very high error noise factor.
For some references on this program, here is a book by the initial scientist who inspired it: Limitless Mind: A Guide to Remote Viewing and Transformation of Consciousness One by its long term program manager: Anomalous Cognition: Remote Viewing Research and Theory and one by its most decorated RVer: The Stargate Chronicles: Memoirs of a Psychic Spy
So mysticism needs to be taken with a grain of salt I AM a practicing mystic. But I generally don't trust mystical "knowledge". This is an intrinsic failing of the of the mystical process: the methods to gain insight, and the methods to do narrative speculation, or narrative reinforcement of prior beliefs, use the same mental channels. Insights and speculation are close to impossible to distinguish internally.
I use mysticism for helping others, rather than being seduced by the allure of a shortcut to knowledge. Seductive allure, and in particular the special status that "being the true prophet" offers as temptation, is VERY skewing and distorting of one's judgment. The contradictions between the messages of the world's self-declared prophets -- is an effective test case on the validity of the "truth" conclusions their method can deliver.
But apply empiricism to the useful practices, and these can provide knowledge, even if not "revelation-based" knowledge.
If one can cure health and psychological problems of today, by spiritual healing, or past life regressions, these useful practices serve as useful bits of data on how a spiritual realm works. If one can communicate with and interact with lost souls, demons, earth-sprites, and "old gods" like Yahweh, these experiences likewise are data about the spirit realm. I gain my spiritual knowledge from useful practice, not from dogma or revelation.
It is true that part of the source of my conviction of reincarnation, lost souls, spiritual attachments, etc. comes from personally experiencing parts of a past life, doing first person spiritual rescue, and experiencing depossession. But I also know how my own mind can mislead me. Part of my conviction also comes from the utility of regression-based healing, the changed feel of a location for others when its ghosts are rescued, and the psychological benefits of depossession.
Learning by Reading?
Now, one does not have to DO these practices oneself in order to gain this knowledge. Reading about other's experiences is not as personally convincing, but can have the same empirical insight. For example, Joe Fisher's discovery of the dark side of the beings that his channeling circle has taken effectively as their "gods" -- is a useful narrative for anyone thinking at all seriously about interacting with the spirit realm: see this review of Fisher's book The Siren Call of Hungry Ghosts: A Riveting Investigation Into Channeling and Spirit Guides. The shamanistic experiences of Carlos Castaneda, while not quite as eye-openingly calling for a rethink of one's prior assumptions, also suggest a spirit realm that is not all rainbows and butterflies. Reading Robert Monroe, Edith Fiore, and William Baldwin will give some more positive examples of what one can discover about the sprit realm -- although the cautionary tales of Fisher and Johannes Greber are also very useful; see my review of Greber's book Communication With the Spirit World of God.
Final takeaway
The best method I have found for doing practical mysticism is that of Ben Swett; see his article Prayer as a Form of Two-way Communication.
they invite debates and opinions that we try to avoid here
- How often is that actually the case? According to my own anecdotal experience, hardly ever are questions on this site explicitly scoped to be answered from the perspective of a very specific school of thought or author. But I'm open to being proven wrong with concrete statistics.