Are there any pros/cons installing Addons using AddOn-store (UI) vs Code (nuget)?
I remember reading somewhere that the recommendation is to always use Code, can't really think of any Cons using the UI though, besides that you let less experienced Administrators to be able to install AddOns.
I would always suggest installing the add-ons locally/on test server first, since any package/add-on might break the site. Having said that, leaving this decision for administrators might be dangerous, since they might try to install the add-on directly on production without testing.
Therefore, I would always advice using Code option, since you have better control over testing/releases.
Apart from that, when you are installing from code, you have control over what is installed and can't go into "which-environment-has-what-installed" dillemas. You don't need to install after deploying to the next environment, etc.