If you spike the brakes at the wrong time on snow/ice, you will lose control of your vehicle and end up in a far worse situation than if you manually brake.
Please can you explain what you mean by manually braking? At its core, ABS releases the brakes for a wheel when it detects that wheel skidding. Braking is still manual until the locking point. Having ABS enables a driver to slam the brakes and know that won't result in a skid, where a vehicle without ABS may lock the wheels and skid.
The effect when ABS kicks in when braking on snow or ice is for the wheels to maintain slightly (or more) better traction. Sometimes this is undesirable - you'll brake in a shorter distance on loose snow if you lock the wheels - but you generally keep better steering control.
Played around with Vehicle stability Assist + ABS and it takes a lot to lose control of your vehicle. Hard breaking without a hard turn is not going to do it without lot's of ice and a curved road surface.
