The risk is that the glue softens, allowing parts to move around. Most commonly this is bridges or bracing. Either they pop loose completely or (perhaps worse) they move and then the glue hardens again when the parts are in a different place. Somewhere around 140 degrees F (60C) is the danger point, depending on the glues used, and I'd want to keep my uke a fair way below that temperature. Inside a car (including the boot/trunk) can easily get this hot.
Humidity swings can lead to cracks, and salt in the air might corrode tuners (but probably requires repeat exposure).
If you can work out how to avoid these then you'll be OK, but the heat problem might be the hardest (unless you're from the UK where there is currently no risk at all, as it's just far enough above freezing for that not to be a further problem).