I am driving a late 2014 model so what I write may not be applicable to the newer models mentioned here. Also the ICE must have been running about one minute before it will switch to parallel mode.
As I mostly (99% of the time) drive in EV mode I am weary about demanding a lot of power/revs from a cold ICE, so I don't 'floor' it unless it has been running a while.
Parallel will always be more efficient than series so if one wants low fuel consumption, you want to achieve that 75km/h switching point ASAP (where safe) and then lift the throttle back where in my case I can keep it in parallel down to 65km/h (speedo shows 3km/h too high), so 62km/h, nobody will book me for that in a 60 zone.
Accelerating from that low speed will drain from the battery rather than switching to series again unless you floor it.
However I always try to drive pure EV mode at speeds up to 80km/h and only press the charge button when I am over or know I will be driving longer at 80km/h so I can avoid the dreaded series mode.
Now when you can't avoid series going up a steep hill, I find it better to switch to battery save rather than charge to avoid racing the ICE.
This can't be good for the engine nor for the battery when it is revving that high.
When I switch to save it runs quiet and I feel I have the same climbing torque.
I don't know if this is an error just with my car or a general design issue that the engine overworks itself by achieving the power for the hill and energy for the battery charge but it sounds so wild like the cylinder head is about to blow in the charge setting.
This is a vey steep hill, maybe 30˚ at times going up with 60km/h.
This era of thoughtless consumption must end so we can encourage a world of creative geniuses rather than consumer idiots.