Actually, they are not quite the worst. That dubious distinction goes to the Bachmann EZ-Track turnouts. I rated Atlas "Snap Switch" turnouts as second worst in a field of seven common brands. My ratings are at the end of the "All about turnouts" file attached below.
Atlas "Snap Switch" turnouts (not to be confused with Atlas "Custom Line" turnouts) have a number of problems. They have a long history of derailments. The reasons for the derailments are built into the turnouts.
The point rails are quite short, which makes for a somewhat jerky transition onto the curved route. There is actually a pronounced kink between the point rails and the closure rails. The point rails are also attached by bent tabs (in the N-scale version) and rivets at both ends of the HO-scale version. Both pivoting systems are a bit sloppy, and let the point rails lean from side-to-side. This can get the point rails out of alignment with the closure rails that they feed into. Other brands tend to use rail joiners as pivot points and this makes their point rails a lot more stable.
The reason for the super loosey goosey point rails, and throwbar, is the weak Atlas switch machine. It does not produce the power of most other switch machines. Its also susceptible to coil burnout unless a CDU (Capacitive Discharge Unit) is added to protect the coils.
One of the biggest derailment-causing parts of the Atlas Snap Switch is the flangeways. Both the guard rail flangeways, and the frog flangeways, are too wide, and too deep, to meet the specs of an NMRA gauge.
The overly wide guard rail flangeways don't do their job of guiding wheels to the correct side of the frog point reliably. As a result, some of the wheels under a car may try to take the wrong (unselected) route through the turnout, which of course, derails the car.
The excessive depth of the flangeway causes wheels to drop into them, and then be dragged back up when they encounter the frog point. This makes the cars rock side-to-side and up & down as they pass through the turnout.
I have had three N-scale snap switches actually shake their built-in switch machines to pieces. This was do to a weak joint between the top & bottom plastic castings of the switch machine. Aggravated by the "slam bang" motion of the twin-coil switch machine.
Atlas snap switches also have plastic frogs, which cannot be powered. (Unlike the Atlas Custom Line HO-scale turnouts, which have powerable Metal frogs.) Not a deal breaker, but it can cause stalls of locos with very few wheels picking up power.
Most of these problems can be fixed. (The files "Improving Atlas turnouts" explain how.) However, most of them can also be avoided altogether, buy simply using better turnouts, Like Peco, or Micro Engineering.
Traction Fan
