@Museek The "in-character" answer is likely that they feel he could be compromised by foreign governments, mostly because of his son's investments and general drama. Potentially "payback" for the whole GOP/Russia narrative.
I don't advocate their viewpoint, but I think that's the linchpin of their argument. It's why they meme about the laptop and iPhone so much.
I know, there's holes aplenty. A lot of their theory is based on assumption and conjecture - stuff like, "Biden isn't hard on China because he doesn't want to endanger his son's investments," or "They're using his son's personal life as blackmail."
No hard, factual evidence. Mostly just conjecture fueled by pettiness. Pretty limp.