About regen, you need somewhere to dump the energy otherwise your DC bus will go excessively high. In diesel/electric locomotives they use massive fan cooled resistor banks, for yours, if battery fed you would need a safe means of getting the energy back into the battery.
Regen is only dangerous if you are operating the machine at a speed that produces a BEMF higher than the battery voltage. In a nissan leaf for example I think its ~120kmh.
Below that rpm, you won't have an uncontrolled energy flow situation, since you can clearly define the maximum regen current limit and the act of regen requires a well coordinated firmware effort. Without PWM switching no current flows into the battery. (attached is the GUI to set this)
If you are above said rpm, you better be prepared for some fireworks, because in a fault situation the motor will dump an uncontrolled amount of energy into the battery. If the powerstage and battery can handle that current, and the vehicle and passengers can handle the huge deceleration, no worries! At least the battery will clamp the voltages on the system to safe levels until speed decreases to BEMF=Vbatt.
If not, you have to embrace the possibility of blowing the battery fuse or tripping the main contactor... now you exchange uncontrolled currents for uncontrolled voltages and the IGBTs will have to endure whatever BEMF voltage the motor generates. If your BEMF at 100kmh is 400V, then your BEMF at 200kmh could be 800V. You will *really* hope the designer chose at least a 900V IGBT. If he chose a 650V device you will have instant, regretful fire.
The BEMF comes from the magnets, thats why Tesla can use 650V IGBTs, an undriven ACIM motor won't produce any BEMF, even if its spins at a bazillion rpm. And when they switched to permanent magnet motors, they now needed an extra voltage buffer: enter wide bandgap SiC mosfets. Beside havin less switching losses, they can easily achieve 900V rating, so the introduction of PM motors and SiC drives are tightly coupled.