When I first built these simple, top entry, false bottom bags, I thought they were going to be limited to colder temps because they lacked ventilation. However, for my personal temperature regulation, in many different climates and scenarios, I have not found too much limitation. I’m still looking, but so far I have found there to be plenty of ventilation through the top. There is also the false bottom that can be spun around to let heat out, but honestly I’ve never used this. If it’s so warm that I can vent enough through the top, I just get out of the bag and drape it over myself as needed. I guess if I knew that it was guaranteed to be warm every night, I would probably choose to bring a lightly insulated quilt, since it is much easier to ventilate. You don’t have to get in/out. You can just stick a leg out or whatever. However, if it’s not always going to be sweltering and I’m going to need insulation, I always choose a FB bag.
I totally get that it might not be for everyone, but just wanted to outline the reasons why I prefer them.