I think, if we can agree, we can pin the date of “modern wars” down to the first world war. (Not discounting the similiar, peripheral wars around the same time, but let’s say the start of the 20th century)
Positing those dates, many horrible things have been introduced to modern warfare. Airplanes, tanks, incendiary bombs, rifles that can kill from two and a half kilometers away, fragmentary AP mines, the overhanging chance of chemical, viral and nuclear retaliation and tactics that apply the above; “Scorched Earth” and “Shock and Awe.”
Most of those things existed in rudimentary forms in pre-modern warfare, but their efficience can hardly be compared to the current incarnations.
With those in mind, I think medieval-and-onward warfare were a lot harder on both the population and the soldiers than modern warfare has been, but I think modern war has the potential to be far worse again, by several orders of magnitude. We simply haven’t experienced two capital modern states with the technology go all-in, yet.
For the civilian populace, it’s always been harsh. In pre-modern warfare, the least of your worries would be that the armies would both lay claim to your crops. In some cases, they’d also burn the crops down in a swathe behind them to ensure a following army didn’t get supplies. Women were usually very much at risk of rape, robberies were commonplace, you could get killed indiscriminately and if you were very much lucky, you wouldn’t get drafted. At the very least, you stood at risk of losing your entire livehood.
In modern warfare, the armies are more self-contained, and usually won’t forrage for local food unless badly pressed or in for the long haul. The civilian populace usually suffer under reprisal attacks for giving their invaders or freedom fighters aid, and civilian infrastructure suffer greatly under the destructive nature of explosives. Roads, civic buildings, vehicles, etc. The chance of getting murdered or raped out of hand is diminished, but still very much there, depending on the origin of the army. The worst part of the civilian suffering is usually the refuge camps, which still suffers from spreading sicknesses, bad water, little food and poor housing.
For the soldiers, well, I dunno. I haven’t fought in any modern wars and the only experience I have with medieval warfare is a few years of re-enactment. That said, I’d take my odds on a sword, shield and armour against a guy equipped with the same, with people I trust at my side, any day. I sometimes freeze up at the thought of the helplessness of the modern soldier, who can be picked off from a mile away and never see the shooter, get bombed into orbit or have to face children with guns. Even the thought of house-to-house warfare makes me want to spit.
That said, there is more to be said of being on the winning side today than there was before, for the soldier. A good chance of coming home, for one. Psychiatrists, for another.
I have to admit, I don’t know the numbers, but I’d imagine the losses would be worse in this day and age, if the two wars compared went on for a similiar period of time, simply because there are more people in the world.