Pests can't become resistant to them though. Same for oils and alcohols, always kills, but can be dangerous to the plants.More sophisticated poisons are usually safer for the plants and may even be long-lasting with systemic or translaminar behaviour. Pests may become resistant though, and mites are some of the fastest-breeding pests so they become resistant fastest. If you ever spray and it seems like it did no good, use something different. You'll probably need to spray more than once a few weeks apart to eradicate mites, so you could spray with a couple of different things. Don't spray one day and then panic when you see something moving the next day. Mites may take several days to die depending on the poison, and even dead mites might look alive for a few days after that.
The Bayer spray wouldn't be my first choice against mites, but it seems to be one of the most readily available products. Most insecticides, except for the soaps etc, are not active against mites. Check the label on whatever you buy, and always be prepared to try something different if it doesn't work.
For flat mites, keep an eye on any plant that has been infected. Not every day because they breed slowly, but every week or two in summer, every month or two in cooler conditions.