I bought a second-hand 18U rack off a chap who was clearing out an office, and I put it in the garage because that's where the space was. It seemed sensible. It is, in hindsight, the single decision I've spent the most time quietly regretting.
The garage is unheated and uninsulated, which means the temperature swings twenty degrees over a day and the humidity does as it pleases. Cold metal plus warm exhaust air gives you condensation, and condensation near electronics is exactly the thing you don't want. I've since added a cheap hygrometer and a small dehumidifier, and I check the readings far more often than is healthy.
Then there's the dust. A garage is a dusty place, and rack fans are extremely good at pulling dust through everything and packing it into heatsinks. I'm cleaning filters monthly that I expected to touch twice a year.
None of this is fatal. The kit runs, the noise stays out of the house, and I've not lost a drive to damp yet. But if you're tempted by a cheap rack and a spare garage, price in a dehumidifier, decent filters, and the time you'll spend worrying. A cupboard indoors would have been the calmer choice.