Of course judicial systems vary with countries, but I'm not aware of a jurisdiction where only the word of the victim is enough to convict somebody (who denies the act).
This is possible in the US. Imagine someone rapes a nun, and she testifies to it. She identifies the defendant, and in his testimony he denies it. If the jury finds her testimony more credible — to the extent that there is not a reasonable doubt as to his guilt — they can convict him.
Of course, it helps if there is corroborating evidence, but there is no rule that says one cannot be convicted (in the US) based on a victim's testimony alone. It just doesn't happen often because usually there is some sort of corroborating evidence of a crime, and because it's hard to eliminate all reasonable doubt without any.
So, it is no different than for any other sort of accusation lacking corroborating evidence?
Is there evidence that this is more likely in sexual assault cases than others? Your post here [1] (and the post you are replying to) seem to suggest that it is actually less likely in these cases.
I think all cases that are based only/mostly on a victim's statement are very difficult to prove. In my other comment, I was pointing out that sex-based cases are more likely to be based mostly on a victim's statement than other cases.
Evidence is evidence. Witness testimony is evidence, photographs are evidence, video is evidence, dna is evidence. All evidence has credibility, which can be challenged.
Every case is different: witness testimony can be far more credible than photo evidence, dna evidence can be less credible than video evidence.
There is — as far as I know — no legal system in the world that measures the validity of a case based on the amount or type of evidence. You can absolutely be convicted on witness testimony alone in the US. That’s very unlikely but entirely possible under the law.
Of course, it helps if there is corroborating evidence, but there is no rule that says one cannot be convicted (in the US) based on a victim's testimony alone. It just doesn't happen often because usually there is some sort of corroborating evidence of a crime, and because it's hard to eliminate all reasonable doubt without any.