Can hand brake levers be pulled hard enough on a comfort/cruiser handlebar? My bike currently has a straight handlebar. Both the front and rear brake are operated by hand, with a brake lever on each side of the handlebar. This allows me to pull them very hard, which is good.

I consider changing the handlebar to a comfort/cruiser handlebar. But I worry that because the ends are tilted backwards (towards me), this makes it more difficult to pull the brake levers hard enough. Is this the case in practise?

Comfort/cruiser bikes usually operate the rear brake by foot, hence they don't have this problem to the same extent.
You will have no problem with the brake levers. The force you apply to the levers is between your fingers on the lever and the palm of your hand on the bar grip, so the angle of your hand with respect to you body does not matter very much.

If you want to convince yourself, sit on your bike, turn the handlebars slightly so the bar is at an angle less than 90° to the axis of the bike and squeeze the brake lever.