Bingo. The only realistic risks come from deliberate user action. If you only install apps and emulators, there is no risk. If you want to install system tweaks, then I also have good news for you. Jailbreaking installs something called the Cydia Substrate. This substrate runs above the system. The complex stuff - theming / mods / resolution tweaks - all run on top of the substrate. If something goes wrong, you can hold one of the volume buttons as the phone boots to activate a "safe mode" that disables all of the tweaks. Now, as for your last comment, that gets a little tricky. Sure, I'd imagine that installing a jailbreak does increase the chance that malicious third-party websites could do something bad. However, I don't know of any examples of this actually happening in the real world. If your phone is full of sensitive government or corporate information, sure, don't jailbreak, play it safe. For an average user in the real world, just don't worry about it. It's not a real problem, its a theoretical one, and the risks are very low.
Yes, that is a downside of having a jailbroken phone. You will be months behind the newest iOS version. And smaller in-between version don't get jailbroken at all. And no, jailbreaking is very easy. All you have to do is follow a guide. Best site with guides: http://www.iclarified.com/jailbreak/
