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In summer, it depends on where you live and what the temperature is like outside. If it's hot out and the sun is beating down on your apartment - yeah, just opening the windows won't help much. But here's the thing: an intake ventilation system doesn't actually cool the air, it just moves it around. If you turn it on at noon when it's plus 35 outside, you're just pushing hot air into your apartment, and circulating that doesn't do much good.
Window ventilation works differently - if you time it right, that is. Early in the morning and evening, when it's cooler outside than inside, open windows give you way more of an effect than running the system during the heat of the day. I used to do it like this: first thing in the morning I'd open all the windows wide for about 20-30 minutes, then close them and draw the curtains. The air in the apartment would cool down by at least a few degrees.
If you're choosing between these two options, ideally you'd have both: the system is great for continuously bringing in fresh air during the shoulder seasons and cool days, but during summer heat, smart ventilation saves you more - early morning, at night, and in the evening. Running an intake system in 35-degree heat is just a fan pushing hot air into more hot air.
Don't rely solely on open windows if it's hot outside - the air simply won't circulate, especially on a windless day. A supply ventilation unit is way more reliable: it actively pushes fresh air inside, cools it if needed, and air exchange happens constantly regardless of the weather outside.
It's not about the system itself, but the temperature. If it's hotter outside than inside your apartment, the intake system just sucks in hot air - it's useless. Open windows don't help in this case either, as someone already pointed out above.
In summer, the time of day matters way more. If you open windows at night and early morning when the street is cooler than your place, natural circulation works great. You close everything during the day - and the air in your room stays cooler. An intake system only makes sense if you're ready to cool the incoming air, but that complicates the system and requires more power.
So in the end, a combined approach wins in summer: night ventilation through windows plus daytime protection (blinds, curtains). An intake system without cooling in the heat is just a fancy fan that pushes hot air from outside to inside.
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