A fellow-student in about 1980 complained of his poor brakes. When a friend and I examined the bike, we found that his suicide levers hooked into the main lever between lever and housing. This reduced the travel of the main lever. Remove the suicide levers and hey presto, good brakes.
I was told that they were invented for coasting down alpine passes, where the requirement was for gentle braking over many miles. They were totally unsuitable for emergency stops for all the reasons above, but the industry sold them as "safety" levers, which got you into the habit of using them when you shouldn't.
Fortunately the mountain-bike revolution killed the market for the things.