On a a bike, every spot has its own bag: the saddle for tools, the frame for dense items, the handlebars for what we want to reach while riding, and the rack for carrying volume.
On the bike, the weight carried on the back tires you out and makes you sweat. The rule is simple: whatever can go on the bike goes on the bike, and the backpack for bike is only for the rest, on short trips or when mountain biking where the bag must stay with the rider.
Each location has a purpose, and mixing them disrupts the balance of the machine.
A pannier bike backpack is useful for mountain biking and short urban trips: you get off the bike with your stuff, without having to take anything apart. For bike touring, the opposite is true. A hybrid bag that attaches to the rack and can also be worn on the back is a good compromise for mixed journeys.
The backpacks bike designed for mountain biking can be recognized by three features: an integrated water pouch, a chest strap and a belt that keep the bag from moving during descents, and a ventilated back. The volume rarely exceeds twenty litres; beyond that, the bag gets in the way of pedalling. A backpack for bike larger than that is for hiking, not mountain biking.