I would love to see a config option to force barrels to only be placeable facing upright. This config would also disable the rendered placement arrows for barrels.

Is it because of any troubles/bugs with the placement system, or merely personal preference?

The idea is slightly worrisome to me, as the barrel placement flexibility is one of their key features - and I can imagine modpacks enabling this by default, or something.

I prefer the simplicity of a vertical only placement, and I'm also not a huge fan of the arrow render, but I wouldn't know a better way to represent the target placement.

I see.

Unfortunately, this is out of the scope for the mod as-is. The concerns of modpacks enabling this by default - and robbing the mod of an important component of its barrel design - are too great.

I might consider it if I introduce a system for client-side configuration which cannot be copied over blindly by modpacks, but for now... sorry. (You can probably achieve this with a client-side tweak mod, though, somehow.)