Variability of timing is another one that is difficult to get the head around.
Why do instructions tend to give such a wide range of cooking times for sous vide recipes? As @AlyssaWOAH suggests, thickness impacts cook time more than weight does. A thicker piece of meat will take longer to reach temperature equilibrium than will a thinner piece.
When cooking a steak, the time range is usually given as between 1 to 4 hours. A rule of thumb for calculating how long to achieve equilibrium is half an inch per half hour. So, a 1" thick steak will take about an hour to reach temperature in the middle. (Yes, the temperature travels from both sides of the steak. It travels at about half of the rule speed from all sides.) This would be the minimum time. The maximum time given is related to food safety. As steaks are often cooked below pasteurisation temperature (54C/129F) 4 hours is the maximum time that they should spend in the cooking bath.
It's often stated that you can't overcook with sous vide. This statement is both true and not quite right at the same time. Specifically I'm talking about proteins as I've not experimented enough with vegies yet.
As you're working at temperature equilibrium the product can never get higher in temperature than the water bath. So, in this respect it is true that you can't overcook. The temperature you set your water bath will be the temperature of your product.
Where the overcook potential does come into it is texture. As temperature selects the done-ness of a piece of meat, cooking time selects the texture. This, to me, is the real beauty of sous vide cooking. You can take a cheaper secondary cut of meat, which is has the amazing flavour only possible in a well worked muscle, and cook it to the buttery tenderness of the most expensive piece of filet steak while still having it done to your favourite medium rare.
Tenderness is achieved by the breakdown of collagen. At 54C/129F this will start to happen about 6 hours into the cook. That conversion will keep going until all collagen has been converted into gelatin. This is where the 'overcook' possibility comes into it. While you can achieve the most delightful textures, anywhere from the 'chew' of a good steak to the fall apart texture of a traditional braise, it is possible to take the conversion process too far and finish up with an unpleasant mushiness. Experimentation and experience are needed to know when to stop to achieve the texture you desire.
It's a good idea to record your experiments so that when you find the perfect time and temperature for a certain product you can be sure to achieve it again and again.