The most common risk is not properly protecting against contamination of the reservoir or not bleeding the system and having air or water in the line.

Air will compress and you could have brake failure. Also water, less common but happens, as the brakes heat up, can boil into gas and then also again you have loss of pressure and brake failure.

This is why I comment technique for race cars, which always have the risk of possibly boiling their brake fluid, will give a slight pump to the brake before coming up to a corner to pressurize the master cylinder and make sure that it does have a little bit of pressure pushback While you still have a little bit of space to react and ditch if something goes wrong

I would assume "if you don't put them back together right, you can't stop, and cars are deadly enough with functioning brakes"

In addition to the other answers, brake fluid is toxic and removes paint.