You're right. Most of the updates I've done on a variety of Linux boxes have not caused any problems - even upgrading from different OS versions (e.g. Ubuntu 12.04 -> Ubuntu 14.04) have proceeded fine. There have been a few occasions where Grub has got confused or distribution upgrades have failed due to LD_LIBRARY_PATH being set etc. However, every OS has had issues with bugs and updates.
You shouldn't need to make a bootable USB as typically, you'd have the media that you originally installed the OS with (DVD or USB or network install). If not, then it's easy to borrow someone else's system long enough to download and burn another copy of it (unless you're in Windows OEM hell).