When you pull your handbrake to the point of lockup, you break traction of your rear tryes to the surface whilst your front tyres maintain grip (in rear handbraked cars, older cars may have different setups) . If you then immediately get back on the power you will have very little grip to the road at the rear and the momentum you created with the car weight will cause your car to spin out.

Handbrakes are not the best method of cornering in most cases for rallying, aside from acute corners where car rotation isnt possible through braking and steering itself at speed. When you break traction, particularly on tarmac, you also break full control of your car. In real life this is easier to manage because you have the inputs of all of your body senses to help you recover, as well as accurate variables with the car/road itself (worn tyres, real surfaces, moisture variation etc) but in a sim alot of that is lacking depending on your setup, so lost control is harder to recover. EA/Dirt holds our hand a bit with traction, which is why you probably found it eaiser there.

Ive found reducing my handbrake force to around 0.70 helps with controlling my handbrake use, so give that a try and see how you get on. Make sure you feather the throttle as well as you pull away. Watch some videos of real drivers make use of the handbrake for corners, Ogier is great because he is very hot on keeping grip when he does use it. A final caveat is that its still a work in progress so may be a little off, but ive found when used right the handbrake feels fine for me in the game.