Login form below fold on Wiktionary and possibly other projects. That suggests this may decrease login rates (it did when the submit button dropped below the fold for Special:Mobileoptions)
Visit https://en.wiktionary.org/wiki/Special:UserLogin?mobileaction=beta&useformat=mobile which forces the use of the core login form to see this.
On the short term we should hide this big chunk of text on mobile screens (either in UserMerge extension or MobileFrontend).
On long term UserMerge will need to be a bit more clever with its placement.