Akismet is a paid service and their apis are tailored for comments. An advantage with comments is that you can just mark as spam if some contents have dubious links or keywords.
An issue you have in many forms (e.g.: login form) is that there is limited data to decide if it's a real user or a bot.
An issue you have in many forms (e.g.: login form) is that there is limited data to decide if it's a real user or a bot.