Have a Linksys running DD-WRT and not a problem. Bought it off a friend who had used the router for 2+ years without a problem. Guess its all luck of the draw.

I've installed thousands of feet of cable, exactly zero feet of it outdoors. I can't vouch for it'* moisture resistance.

However, you could install some weatherproof conduit on or under the ground. Again, outdoor cabling is not my specialty, but PVC pipe should suffice if properly sealed.

In either case, do not exceed 330 feet of cable without an active (powered) signal repeater, and avoid any embedded power lines and sources, as regular Cat5e is unshielded.

I'm sure it'll be fine for a while

Yeah, it will be fine, so long as some animals don't start chewing through it.

Oh yeah, I've done temporary installs through active windowsills. Cable'* pretty resilient.

I'm just more worried about some animal chewing through the insulation and letting moisture in. My old roommate had pet rats who did a number on my laptop'* power cord when I draped it over their cage.