There's a lot of stuff you cannot do on the mobile forums. For example editing your posts or adding links or spoiler tags.
And even liking other's posts is sometimes rather difficult, as the button is very tiny on the mobile version and you frequently keep clicking in the person's picture, getting linked to their profile instead of liking their post.
Also you cannot easily go back to the previous viewed forum section. If you use the back button it'll show up as not read, so if you want to go back you gotta go through forum start page and navigate all the way back to the sub forum you were viewing.
You cannot zoom the pages either, so things like viewing pictures that are sometimes small are kinda impossible.

Long story short, the ffxiv forum has a pretty bad mobile version, which should be reworked. I can't be the only person who views these forums mainly on mobile, so a little quality of life improvement would be much appreciated.