There are 2 burdens of proof in trials, 1 for civil trials, and 1 for criminal trials. Civil trials require a burden of proof "beyond a reasonable doubt" and it is much lower than the burden of proof required for criminal trials, which is "beyond a shadow of a doubt". The burden of proof you are describing for Kevin Spacey's criminal trial is actually "beyond a shadow of a doubt", which essentially requires reliable eyewitnesses or a smoking gun, as they say.

That said, Spacey also defeated a civil trial in the US last October for a different set of accusations, so there is that 🤷

I can't claim to know the truth in he said/he said situations like these, but common sense would indicate that there's probably some truth to multiple accusations of impropriety. Victims often don't opt to speak out publicly and go to court unless they think they can win and scam artists are rare.