i don’t have any experience with unetbootin so i can’t help try and troubleshoot that part.
if you are interested in trying dd, the command is fairly straightforward, but you definitely need to be careful. it would be something like:
sudo dd if=~/Downloads/name-of-distro.iso of=/dev/sdX status=progress
followed by sync
it is, however, imperative that you choose the correct partition to write to for the second half (of=) of the command because you can overwrite your presently mounted partition if you get that one letter wrong. just this week i was seconds away from wiping out my timeshift snapshots backup drive because i recycled a command from my history and didn’t check of= carefully enough. luckily it somehow caught my eye before i typed in my sudo password.
so this part bears repeating. if you choose to proceed, verify and then verify again 
i use lsblk in the same terminal that i am going to use dd so that the info is right above when i am typing. lsblk output looks similar to:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 223.6G 0 disk
├─sda1 8:1 0 46.6G 0 part /
├─sda2 8:2 0 289M 0 part /boot/efi
├─sda3 8:7 0 182G 0 part /media/user/data
└─sda4 8:8 0 11.2G 0 part [SWAP]
sdb 8:16 0 16G 0 disk
└─sdb1 8:17 0 16G 0 part /media/user/usb-label
i can tell /dev/sdb is my usb because of the size and it’s mount location in /media/user.
in the command if= is where your iso is located so if you need to change that to ~/Documents or /home/user/isos (~/isos) or wherever you store that, you will need to adjust accordingly. status=progress will give you a status bar indicating how far you are in the write progress and sync will make sure that the write is definitely finished before you take out the usb in case you do.
i use dd because i tried etcher and some other methods when i was first starting in linux and had more bad than good results. dd has gotten the job done nearly every time.
even so, i would recommend not using it if you don’t feel comfortable doing so and have a good backup handy.
just because it also exists, there is grub 2/iso boot if the usb ends up being the problem.