Infopath can be used to create workflow start forms, however it does not have the nwcontext xml resource file necessary to use an item's metadata, unlike task forms customised using Infopath.
Whilst variables such as approver comments are not applicable, it would be very useful to provide the basic fields such as item ID, creator and item url.
I've been told by Nintex support that this discrepancy is intended functionality and to raise the request on here. Infopath is still a much stronger form designer than Nintex forms, for things such as multiple data connectors or unique values in dropdowns.