Is regenerative braking always suitable for an EV regardless of battery chemistry?
Assuming the cell is sitting behind some basic controller--what happens to back-EMF from a large (200W+) motor if the cell is &quot;fully&quot; charged and the BMS shuts off the back-EMF transient?

200W for an electric vehicle is rather small, if not to say microscopic. A Nissan Leaf from 2010, if I'm not mistaken a relatively small commercial EV, has around 80 kW, i.e. the 400-fold in power.
You're right, there's a controller, and a bidirectional AC/DC converter between motor(s) and battery(ies).
That converter's job on recuperation is to rectify the voltage from the batteries; since its bidirectional nature requires this rectification to be synchronous, the controller will also control the duty cycle with which that happens, and hence the voltage on the DC and thus the charging current
In case you brake with a full battery, only little energy will be allowed to flow through that converter; most will either simply left in the AC circuitry (i.e. like disconnecting the motor) or sunk into a resistor.
If you need the decelleration, then the converter will instead of charging discharge the battery to cause a current opposing to that which the turning of the motor causes.
For street-legal western EVs (i.e. not golf carts or similar), you underestimate the amount of cost that flows into the control and build of the converters. These things don't &quot;blindly&quot; let power flow between motor and battery.