Are there any downsides to having shares? Am I liable for company debts?
No, there is no downside to you being given shares in imutual. They are free, fully paid-up shares and you are not liable for any of the company's debts. It's just the same as owning shares in, say, Facebook - if Facebook went bust, then your shares would be worthless but you wouldn't be responsible for any debts the company left behind
If, for some reason, you don't want to be a shareholder or receive free shares, you can opt out and still earn cashback.