The most common failure is extreme imbalance that eventually drives a cell to failure. There are 168 individual cells in your Prius. These are contained in 28 modules with six cells each. They all respond slightly differently to use and age. Eventually, they get out of sync with one another until they no longer function well together, and one or more cells fails catastrophically.
Additionally, while a robust design, there is somewhat uneven cooling within the pack. The modules in the middle that are surrounded by a large mass of modules on either side, tend to lose their capacity more quickly than the modules towards the ends of the packs.
Here at Phoenix Hybrid Batteries, we have developed a process that reconditions the individual modules and restores them to near optimal conditions. We're not going to tell you some number like 90-95% of their new capacity because that is very misleading. Those numbers are achieved with very low currents when compared to what they see in the car. To ensure the best results, we use very high current in our testing - representative of the currents the modules experience in the car. Brand new modules won't meet 90-95% of their new capacity in our tests. There are several other pass/fail criteria that determine if a module is suitable for use. On most packs, we find that about 20-25% of the modules within the pack do not perform well with their pack-mates for a variety of reasons. This is not because they are bad, but it is because they are not well matched/balanced.
This disparity is why reconditioning a pack with simple module replacement has shorter lived results than a completely matched and balanced pack. We utilize strict criteria to ensure all modules perform optimally with their pack-mates.