I am trying to identify Nintex Mobile current limitations on iPad. for the moment I can't :
- have any custom style on my field and labels
- display specific message using validation rule (the message displayed is only "before submit this form, you must correct validation issues")
- use any lookup field
If you have any other features that are not working using Nintex Mobile or any remarks on this ones please comment
For formatting, please see this post on available options: Nintex Mobile HTML formatting support
For which controls or options are available, please see this post: Mobile Apps - Controls used in Mobile Form Design
Also, Mobile Apps are ever changing. Newer capabilities are added many times a year. You can view new releases here Product Release Announcements and also sign up for email notification.
Hope that helps!