Upgrading from 4.7.12 to 5.1.2 on Wordpress
Not particularly. We have nursed upgrades through from 4.4 and 4.6 (yes we inherit some shockingly old systems occasionally) without significant problems. On that scale 4.7.12 doesn't seem to be 'such an old version'.

If you backup code and db and give it a try and you hit issues then just roll back and come back on here with some details of problems you encounter.

(Edit to incorporate Ramesh's helpful comment)

Refer this link docs.civicrm.org/sysadmin/en/latest/misc/switch-servers also you can refer to docs.civicrm.org/sysadmin/en/latest/upgrade/drupal7