Dale Allison, The Resurrection of Jesus: Apologetics, Polemics, History (London: T&T Clark, 2021), 76:
For the critical historian, then, 1 Cor. 15:6 amounts to disappointingly little. Many who find it impressive would surely brush it aside were it a claim about Kali rather than Jesus, or were it found not in the Bible but in the Vedas. We know far more about the miracle of the sun at Fatima, when a throng of thousands purportedly saw a plunging sun zigzag to earth. But what really happened there remains unclear, at least to me. We also have decent documentation for an alleged appearance of Jesus to about two hundred people in a church in Oakland, California in 1959. Yet the evidence – which outshines Paul’s few words – leaves one guessing as to what actually transpired. It can be no different with the appearance to the five hundred. When the sources say little, we cannot say much.
The Amateur Exegete 
Although I’m at a disadvantage having only read English translations, nevertheless, 1 Cor 15:5-8a always “sounds” incongruous to me, like a later interpolation. It sounds out of place, desperate even, and not in keeping with the rest of Paul’s writings about [his] Christ. It strikes me as a copyist’s later attempt at justification of the resurrection. We do know that when some theological point was being made during the Church’s history, a manuscript would often suddenly be “discovered” which supported it. Am I correct that the majority of surviving, complete, NT manuscripts are from the 800’s CE? If so, we would *seem* to be on very shaky ground being absolutely certain of what was originally written. I’m happy to be corrected if I’m way off base on this.
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I’m never a fan of interpolation theories and to my knowledge there is no manuscript of 1 Cor. that lacks this section. The earliest copy of 1 Cor. that contains this passage is P46 that is dated to the beginning of the 3rd century CE. (We also have majuscules from as early as the 4th century as well, plus patristic witnesses.)
The 500 witnesses thing sounds like the kind of throw away but “look here” comment I’d expect to find if an ancient author was trying to casually bolster an argument. It probably impressed a lot of people, just as it does today. But, as Allison notes, it’s worthless for a historian trying to figure out what actually happened.
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“Many who find it impressive would surely brush it aside were it a claim about Kali rather than Jesus, or were it found not in the Bible but in the Vedas.”
Exactly. There’s a natural human inclination to accept claims from our own religion that we wouldn’t entertain from the other fella’s.
Another comparison I like to use: The documentary and archaeological evidence corroborating Herodotus’ description of Greek divinities smiting marauding Persians with lightning and rockslides to protect Delphi is as good–in some respects better–than the evidence for the Red/Reed Sea miracle in Exodus. Crediting the latter while dismissing the former is an exercise in special pleading grounded in a theological commitment, not intellectually consistent epistemology or historiography.
The Assyrians, Babylonians, Celts, Egyptians, Greeks, and countless other peoples of antiquity promulgated narratives involving a variety of deities, demigods, spirits, transcendent realms, giants, healings, resurrections, divine interventions, visions, etc., etc., etc. that most modern Christians discount as products of the human imagination and penchant for storytelling. Yet somehow the ancient Hebrews and Christians alone were immune from this tendency? Out of the myriad cultures across the ancient world, only they perceived and wrote the straight dope with clinical accuracy? Not very likely, by my reckoning.
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