Good point eric, that elastic on the chest strap makes a huge difference. If a hydro pack is setup right it shouldn’t make riding more difficult.
Waist packs, on the other hand suck bigtime! My first hydration pack was a waist pack (way back when hydropacks were first becoming popular). If they’re designed to hold enough water for a decent ride they are really hard to keep in place. They work ok for hiking, but for biking, muni, etc. They jump around and mess up your rhythm. If you cinch them down tight enough to keep them in place, they make it hard to breathe.
If 2 or 3 litres is distributed well on your back and you have good waist, sternum and shoulder straps you hardly notice it. Especially if it’s there every time you ride.
Then again i’ve cleared logs with a gallon of iced tea in my backpack 
but that is my honest opinion.