One can never predict the reasoning behind such reviews. Some may simply be attributable to having a problem they neither understood or knew how to fix. I’m not personally aware of any justified reasons, but certainly, once the ball starts rolling, there are many who like to join the game.
As you may already know, the AUR is not officially supported by Manjaro (or Arch, for that matter). The usual recommendation for those who (for whatever reason) want to use the AUR is to switch to the Unstable branch, which is the closest match to Arch (stable).
See Switching Branches - Manjaro
The order of updates is important to avoid complication when using the AUR. It is recommended to follow this best practice method:
1. Update regular repository packages first with:
pamac update --no-aur or sudo pacman -Syu
2. Then, if you then wish to update software from the AUR:
pamac update --aur
3. And if you also use flatpak…
flatpak update
It is also recommended to re-build AUR sourced packages after every normal repository update, to help ensure that respective dependencies are current.
The “Live ISO” environment will launch the same installation routine. There is effectively no difference between selecting “Install Manjaro” from the menu while booting the ISO, or clicking the icon on the “Live” desktop.
There are of course differences between the Minimal and Full ISOs. I personally suggest always downloading the Full ISO to avoid the possibility of missing applications that otherwise might have been expected to be available. That of course is a matter of choice.
I hope a few of your questions now have answers. 
Regards.
The information that follows is designed for new forum users.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit.
To that end, links are provided (below) - Please use them.
Be prepared to provide outputs from various commands when asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Waiting for someone to blindly ask questions is also counter-productive – typically, nobody has a 
at their disposal – Instead, please help others to make informed suggestions, based on information you provide.
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly; 
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
inxi --filter --verbosity=8
or the short form (preferred):
inxi -zv8
- inxi man pages (Arch Manual)