Like Li-Mn they are a safe chemistry battery that is unlikely to blow up in your face. However there are two issues with Li-FePo4 cells:
1. Many of them are low quality and have a low C rating, meaning they cannot be safely used for atomizers as the current draw is over their limit. There are two groups of Li-FePo4 cells: those with a C rating of 1 (or less), and those with a rating of 4 and up. The C=1 cells will be over-driven by an atty: a Li-FePo4 14500 battery with a capacity of 600mAh and a C rating of 1 can supply 0.6 amps to an atty. This is too low and it will be over-driven. A cell of 600mAh with a C rating of 4 can supply 4 x 0.6 A = 2.4 amps, which is fine.
2. Almost all of them are 3 volt cells, not 3.7 volt cells, which means they cannot be charged in a normal lithium cell battery charger. This would charge them to a final voltage of 4.2 volts, way too high for these cells, and will damage them. Damaged cells are a good candidate for self-destructing.
Now imagine a scenario where a pair of low quality Li-FePo4 cells are overcharged in the wrong charger, then paired in a 6 volt
mod, then asked to supply 2 amps+. This is a situation where unprotected Li-ion cells (or possibly, protected Li-ion cells where one of the protection circuits has failed) have blown up violently in the face of users when asked to deliver the first draws on an e-cig after coming off charge. It is my understanding that a Li-FePo4 cell will not explode in this situation but it will still fail and de-gas, just not as strongly as a Li-ion. The difference is just in the speed and volume of the gas produced. Maybe if the mod has no vents then things could be tricky.
No battery is safe when abused, but in theory Li-FePo4 and Li-Mn cells will not de-gas so violently that they are in effect exploding - but they will still burn up if mistreated badly enough.
Because of the two Li-FePo4 issues, it is safer to use Li-Mn cells in a mod. But even those cannot escape the consequences of mistreatment. They are very unlikely indeed to burn up without being mistreated in some way - so you need to check the off-charge voltage, use them in a mod with a master off switch and gas vents, and don't jam the mod in a pocket and leave it. The use of PCC cases for generic rechargeables currently has no history or safety record so may not be considered entirely safe at this time.