Once again it's time for me to give my usual words of wisdom about OS upgrades, this is not my first rodeo, I've been doing Mac operating system updates/upgrades since the mid 80's.
1) Do not try to upgrade the current bootable partition when booted from the same partition. Things go MUCH smoother if you boot from a partition OTHER than the one you are upgrading. Hard drive space is so cheap, external drives are available for under $50. Cloning software is readily available, in many cases free, and work extremely well with proven track records. There is no excuse not to have extra Mac bootable partitions, either on your main boot drive, or an external drive.
Either clone your current working partition or create a new one by doing a fresh install. A fresh install of Mojave will easily fit in under 30GB of space. Even if you clone your current working partition, no matter how large it is, you don't have to clone the whole thing, just operating system. Leave out all unnecessary apps. It will fit in under 30 GB of space.
30GB is nothing when you can buy 1TB and 2TB external drives for under $50 on sale from any Office you know what store in Your Town USA any day of the week, or A^&zon, New%gg, BBuy.
Using this method (booting from another drive or partition other than the one I want to update/upgrade) I RARELY have any issues at all.
2) When an app developer hasn't confirmed their app works with the new macOS, don't go upgrading and then expect the app to work, or complain afterwards ,when it doesn't. If a developer says upgrade the app BEFORE upgrading the OS, there is a reason, and don't say 'Oh, they just want to pick my pocket for updates every time Apple changes the OS. The only thing these developers do is just change the version numbers the program is compatible with.' I won't say that SOME developers don't do this, but the vast majority actually have to make changes to their app with each OS, often significant changes, changes that require new development, testing, QA, and so. Apple has a LONG history of changing technologies from release to release, sometimes right up to the last minute. So, when a developer doesn't have a compatible release of their app on DAY ONE that an Apple macOS upgrade is available, it's because even if they tested on beta releases of the OS, they want to be sure Apple didn't change something at the last minute that blows their app up.
3) Don't freakin' upgrade your day to day WORKING environment blindly, if you don't want to lose data. Again, with disk space being so available, clone your working environment, upgrade the clone copy first to make sure all is well. If no issues, great, clone the test partition back to your working partition and move on. NO risk to your working data or setup, NONE.
4) Broken record; backup your data/system before any major upgrade. I used to feel sorry for people who lost their data; no more, after 35+ years in tech, I'm jaded now. No backup, data loss, your fault, not Apple's. It's leading edge technology....and guess what, s&*t happens, even with the best planning. No, Apple didn't do it on purpose, for pete's sake. All those Apple programmers are just LIKE YOU, they have mortgages to pay, kids to put through college, elderly parents they have to take care of...they need their jobs...no, they didn't do it on purpose just to mess with people.
Mojave has fixed a lot of the rough edges that had accumulated since from Yosemite through High Sierra. IMO, it's the smoothest, and feels the 'nicest' macOS since ...oh, since maybe Snow Leopard, which I 'LOVED'. On recent hardware that is Metal compatible, it really moves along, feels and responds very 'snappy'. If you are in a business environment or depend on your Mac for lively-hood, I would still recommend waiting on updating till like maybe version 10.14.2 or something. For personal use, as long as you backup regularly and don't have apps you depend on that need to be updated to run on Mojave, feel free to jump in.