How bad is Whiskas for cats and why?

I have a 9 year old cat and a 7 month old kitten who are both on whiskas atm. I've heard that whiskas are quite bad for cats yet the brand is quite popular.The 7 month one use to take royal canin at first but i took her off of it because it started working out to expensive. Any tips or ideas on how I can improve my cat's diets??