If they have neutral bottle handups I’d err towards using those and using my own nutrition in the form of gels. If bottle hand up…then I generally think taking a minimal bladder and bottle(s) with exact water/nutrition already in them is the way to go. Sometimes ultra running packs are smaller, lighter and fit more aero than anything made for cycling.
Refilling a bottle mid race or asking some volunteer to fumble around and do it for you is a great way to watch your competition get a 2+ minute lead and never see them again, or find yourself behind a pack of slightly slower riders when you don’t want to be.
Downside of pack is its hard to see how much you’re going through, the weight and potentially heat dissipation on your back. Upside is its way easier to drink out of through technical terrain, and it’s nice not having to rely on anyone else.