I've done a science foundation year and and I'm now progressing to my first year of my degree.
Difficulty depends on the uni tbh, all unis structure their course and choose topics differently.
I'd say the content isn't necessarily hard as long as you have done relevant A-levels before you should be fine. I think what's more difficult is that it is demanding and that the timetable is jam packed so you are always working on something, alongside this sometimes the percentage that you need to get to pass to your chosen degree can be high. sometimes it can be 55%, 60% or even 70% etc depending on the difficulty of the course. (there are conditions for each module)
During the maths modules I have encountered some further maths work which I have never done before as I had only done A level maths. However, luckily enough the lecturer was very good in explaining and always put their notes up so I had no difficulties with this.
Also attendance is really important aswell, that can also determine if they allow you to progress to your chosen degree.
If you have never studied A-level maths, or any science (not related to the degree), then yes. It is difficult. However it is possible.