NOTE: Unless otherwise specified, Volt refers to the 2nd Generation Volt (2016-2019).
I wonder if some of that is due to the lighter weight of the Volt, even though it's not all that much lighter. That may also be why the Volt will let you stay in EV even with the accelerator pressed to the floor.
The Volt was designed to be a BEV first and then a hybrid second. This translates into a much "purer" (whatever the heck that means) EV experience. The Clarity is a hybrid first - Honda missed the boat on this one because their FCEV and BEV versions have an electric motor more than powerful enough to not require ICE assistance. The Volt's lighter weight, combined with the 50 KW ICE power generator allows the 2nd Gen (2016-2019) Volt to handle the worst climbs using ICE only. The ICE in the earlier Volts wasn't powerful enough to do this.
What I think really hampers the Clarity in these situations is its unwillingness to use battery power below the set point for extended periods of time. You could have 90% SOC but if you have reached the set point in HV mode then it refuses to use the battery and will even try and charge the battery if it drops below the set point. You can do HV reset (momentarily turn HV off then back on again) which sets it a little lower, but that only helps briefly and anyways you shouldn't have to do that. The fact that people are doing HV reset means the software is not doing what people want. There should be a mode (maybe called HV Power) that you can select where it is willing to use some battery power to supplement the engine for extended periods like during a hill climb. If the SOC starts getting too low then you can always switch it back to regular HV Mode. In fact this version of HV mode could also make life quieter on city streets, for those willing to give up some EV range it would keep the engine off as you accelerate from a stop and then only start using the engine when the acceleration is finished (or tapering off), at that point not as much power is needed from the engine plus wind and road noise will pretty much drown it out anyway.
Hybrid (HV) hold mode is just that - keep the traction battery at the level it was at when the driver selected HV, which means going full electric after descents and recharging after climbs or other surge power requirements. The purpose of this is readily apparent in Europe where some of the major cities have instituted a BEV only requirement for inner city personal vehicles. HV mode allows the driver to get on the highway, switch to gas, and then switch back to EV for those areas. The 2011-2012? Volt didn't have this mode (which GM calls Hold Mode). What Volt owners were doing was putting their car into Mountain Mode, which preserves the battery at a low level and will recharge the car to that level from the gas engine, and then put the car back into normal EV mode for the city. GM responded in 2013 (year?) by adding Hold Mode to address this.
My Volt actually keeps a 400 W buffer at the bottom and taps it to use it's electric motors to accelerate under heavy load, even when the battery is depleted. The ICE will then rev up to recharge the battery, leading to a rubber band effect on the engine sounds. The Clarity appears to keep a smaller buffer for this purpose. Normally an empty battery shows two bars on the battery display but I've seen the car showing one bar. It's when the car is showing one bar that the ICE really gets a workout.
Where Honda missed the boat is by not giving the driver the ability to put the car into HV Charge (Volt Mountain) mode as soon as they leave their driveway, allowing the car to drive electric for a ways and then ensuring the battery is maintained at a factory preset level.