As safe as what you're side loading. Side load a virus and it's not safe. Sideloading an update is actually safer than OTA - you can't lose the connection because someone ran the microwave.
If it's an update, yes. That's the difference between flashing a ROM and running (by sideloading or any other means) an update.
Not if it's the official update and you don't root the phone.
If you don't know what you're doing you could mess up the phone, so always back up
all your data first (in case you have to flash a stock ROM - which will delete all your data - and start again). But if it's a captured update (the update deletes itself after it runs - someone has to stop it from running and copy the downloaded file to someplace safe, like a PC), there are no cons other than the ones in that update - like the bugs in 5.0. Sideloaded or OTA, you're going to get the same bugs.
If the OTA for your phone hasn't been released by your carrier yet, no one's captured it, so there's no way for you to side load it. Don't load files that people claim are the update, wait for the real one. (Look in the
XDA Forums subforum for your phone - that's where you'll find captured official update files.)
(The fact that Google released 5.0 for the Nexus doesn't mean that 5.0 for your phone for your carrier has even been written yet. If you side load a ROM for a different phone, the best you'll do is brick the phone. You could also damage hardware.)
A side note: 5.0 is a bug. 5.0.1 is a bug fix. 5.0.2 is a further bug fix, but it's still pretty buggy on some phones. I'd wait until 5.1 has been out for at least a month to see if there are still enough bugs to not want it. My Note 3 on 4.4.2 still works just fine, and it has no LolliFlop bugs. (And I don't need white screens - they just eat more battery.)