Religious fundamentalism, to be frank, is the drawback of it, and the pushing of simplified narratives (e.g. “The entire Dīgha is fake” and nonsense like that) instead of what I would call “real EBT studies.”
Three-nikāya purism verges on the EBT equivalents of pseudoscience and conspiracy theory, really, IMO. The only people I have never known to be three-nikāya purists all claimed to be arhats or stream-entrants on the internet, so we (and by “we,” I mean “I,”) know it can be a gateway drug to prelest as well.
For instance, five-nikāya purists don’t believe in the monastic vinaya or in EBTs like the Dharmapada, because it’s not in the five nikāyas. In my experience similarly, oftentimes three and five-nikāya purists are the most vocal and angry opponents of rebirth and karma, generally claiming the beleifs to be Mahāyāna or Hindu in origin. I hope that contextualized somewhat what I meant.