I would say they are, if you do them incorrectly - just like back squats.

More in depth answer:

No, the pistol squat isn't bad for the knees, in the same way that back squatting isn't bad for the knees, if they're done correctly.

One of the great ways to figure out if doing the pistol squat will hurt your knees, is to understand if you have any injuries already, weak unilateral leg strength, lack of mobility, or muscular imbalances that one should look at before they try to go into full ROM pistol squats.

Trying to force the body into a position that it can't exert full force in a controlled manner/ control in general isn't a good idea. Below you'll see what you need to do (or not do) if you want to build a safe, healthy, and beneficial pistol squat.

What is the pistol squat?

The pistol squat is a unilateral leg movement that's usually done with bodyweight or other methods of adding load, that is meant to improve joint integrity and mobility, leg strength, and total body balance and control.

How to do the Pistol Squat?

Pistol Squat Progressions

When you're trying to do a stable, healthy, and strong pistol squat, it's important to do proper progressions, as with any movement really. Without having proper joint and muscular control, mobility, strength, and coordination during the movement's entire, pistol squats (as with any movement) can injure the joints and muscle. The information above tells you what you should do to progress throughout the entirety of the movement, to diagnose mobility issues, and build basic leg strength and balance.

What shouldn't you do?

Below you will find 3 ways one can make the pistol squat a painful, jarring, and joint damaging movement.

• Lifting heel off the floor: Squatting on the toes is never a good idea. So, why would a person want to do the pistol squat on their toes. Usually, when the heels come off the floor at the bottom of the movement, this is due to a lack of mobility in a joint. The joint's immobility can result to one overcompensating with other parts of the body such as ligaments, tissues, etc. If you can't keep the heel firmly on the floor during a pistol squat, you should reduce the range of motion and work on control and range mobility.
• Using excessive heel lift to assist: If you have issues keeping your heel firmly on the floor during a pistol squat, you could use a heel assist (plate under the heel, weightlifting shoes). While this will work as a band aid for reduced mobility in one of the joints, it can result in similar issues as listed above, if the lack of mobility isn't addressed.
• Crashing into the bottom of the movement: We have all seen this done, and a good majority of us are actually guilty of it. Crashing into the bottom of the movement (not exercising control over the movement) is never a good idea unless the pistol squat has been trained with strict reps, at a full ROM, without the added bounce, at first. By bouncing in the pistol squat, you are adding extra load onto the ligaments, small joints, and tendons in the knee and ankle. Once you have the coordination and strength, as with the back squat then it is okay to sparingly use the bounce. As long as you understand the load that it places on the connective tissues in the joints.

As with squats, stability and strength, body control, mobility , and proper progressions are necessary to make sure that you have movement integrity and sound joint mechanics. Using "band-aids" or taking short cuts can lead to injury if they aren't addressed.