The idea with Seneca is that you use it regularly, and it will direct you to content you haven't previously done well on in the future. This is called spaced practice, and it's a concept with a lot of evidence behind it. However, with Seneca, this will only work if you use the site regularly. If you use it for a few weeks and then stop, then obviously it won't work as well.
I think the promise of guaranteed grades is ridiculous though- especially in more essay based subjects. AFIAK, seneca just focuses on retrieval of content, rather than the exam skills you will need to get top grades. That's not to say you shouldn't use it, just that you need to be doing past paper questions too and getting used to the markschemes.
For most subjects, it's only the second year of 1-9 grades as well, so there could be some surprises in marking.