Safety is also a consideration. In September 2018 the US Food and Drug Administration (FDA) released a statement warning about the risk of tremors and seizures in animals treated with products containing drugs in the isoxazoline class, which are used to combat fleas and ticks. However, the FDA said it still considers the products "safe and effective".
Combination products can be cheaper than buying them separately. On the other hand, it can be difficult to compare the cost of combined products with separates, because it depends on the ingredients (for example, newer and potentially more effective flea treatments are more expensive than older ones) and the cost of supplying what's missing – there's no all-in-one that covers everything. And that's before you take the cost variations from retailer to retailer into account.