There can noticeable effects of cogging in some direct drive hub systems and parasitic drag due to gear reduction components in geared hubs and mid drives without some sort of freewheeling mechanism. These can make pedaling feel slightly sluggish but still doable.
As noted however the additional weight of e systems are going to be the most noticeable difference when any resistance such as hills, headwinds and getting going from a standstill occur. This applies to all eBikes.
One thing to watch out for is that most e-bikes are geared rather high and have a fairly narrow gear range. This is typically fine under power but you'll find yourself wanting lower gears when you are pedaling unassisted.
There is I think a big difference between a 2015 and 2018, I can ride up any hill around here with no pedal assist whatsoever, just a touch of the throttle, I use the manual mode more than auto and shifting to a lower gear is super easy, simple twist with finger and thumb, I have the electronic shifting on my Aurora so no muscle required. The new one is also 750W and 90NM of torque so I don't think you'd need more. I have ridden mine on flats with no power and its fine but a steep hill would be tough. I don't notice the weight of the bike at all when riding. I also have a Trek with gears and it sure beats all the chain popping and clunking , these new auto hubs are Great!
I'd love to try a Rohloff hub but the wife's Nexxus hub is outstanding for a leisure bike, especially since she never really got the hang of shifting her deraileur equipped regular bike. She also has a completely enclosed chain. Not as nice as a belt but you could eat off that chain after 100mi. whereas my exposed chain needed cleaned and lubed