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A fair point no doubt but I think it comes to not only him being emotionally compromised and as you mentioned (which I actually forgot!) being previously injured, he was so beset on retrieving the whereabouts of Luke and now his lightsaber that it probably amounts to nothing more than an incredible lapse in judgment - something he's plenty familiar with as we've seen. We know Ren is capable, perhaps more so than Vader (he definitely has a bit more of an evil streak to him) but all throughout the movie he hasn't been planted on either side of the force.
Should Rey have posed that much of a threat so early on? It's a little hard to say without knowing more because he was shaken up earlier by her Vader comment, shaken up even earlier than that by discovering there were latent abilities awakening within her while he was trying to force information out of her, and perhaps even more than he just impaled his father which clearly had an effect on him as you could see in facial expressions when he confronted both Rey and Finn.
I don't think it's as easy as "Well Ren was better in almost every conceivable way." because while that's almost certainly true he was torn in too many different directions to assert that.
...I think there were one or two scenes or lines of dialogue that were cut, that maybe could have given more context to the situation, and the development of the character.
I can only think of one concrete moment when Ren's reaction hinted at his shaken psych:
...when he arrogantly tried to summon Luke's saber but it didn't respond to his calling, rather at Rey's - hell, rejection's a bitch y'know.