Although my schoolboy electro-mechanical knowledge is over 50 years old, I don't think the basic physics has changed. All motors need momentum to keep turning between the energy input bursts - even an ICE - so initial movement is difficult.
With a traditional electric motor the central armature has a large mass to keep it turning as it moves from contact to contact but even that can be prevented relatively easily - ever had an electric drill jam? Sure an electric motor can't stall, but the continual inflow of energy is just turned into heat - as we have already discussed.
As your diligence has discovered, I have made this point before, but in relation to actual experiences of PHEV users. I think it might have been Trex who demonstrated the brick/kerb fail but others reported problems on slopes, including slipways. The solution, is to have momentum when meeting the obstacle - I've seen a Mitsu video of the car climbing a 45 degree ramp with no problem but NOT from a standing start. So the usual way of dealing with this is to back up and take a run at it - unfortunately, you may not always have this option, especially on a slip way.
Yes, if you are likely to want to tow a large boat up a slipway, then the PHEV is probably not for you - you need a vehicle with the motor up front well out of the way of the water.