Yah, I was simplifying to a specific speed/force point (and region above max regen).
You bring up a great point that regen can't produce more force (acceleration) than it can produce. A car stopped on a hill has all its braking force available, whereas once in gear it is limited to the motor ouput. (Automatic pointed uphill for comparison, take your foot off the brake and it rolls backwards, but on a flat surface it would creep forwards). So you can have slope that regen can't hold the car in place on.
If regen is open loop controlled by a speed to kW lookup table to provide an approximately constant deceleration force, then regen can only trip the braking limit if it can achieve 0.3G on its own, which it would then also do on flat ground (unless the open loop includes vehicle angle).
If it is close loop controlled to the vehicle acceleration (up to the power limit), then 0.2G of net deceleration requires 0.3G of regen induced force if on a 0.1G slope and it could trip the braking limit. Due to the power limit, a quickly moving car could have less regen than needed to decelerate on the slope, but would slow at a lower speed.
Alternatively, regen could have both a power and speed upper bound (don't slow too quickly). That would also allow more regen force on a downhill than on a flat (clipped due to speed change)