I need a tool to create forms that works both in SharePoint 2016 onPremise and in the cloud.
I must be able to create tabs and in the same time disable/hide and enable/show fields depending on other dropdownfields selected values.
Is this possible with Nintex Forms?
The things you describe are possible with Nintex forms. Tabs are currently supported through the use of built in form rules and the hiding and showing of panels as well as a bit of CSS. The products are very flexible and pretty user friendly. The one thing to be aware of is that there is a product offering for on-prem and a separate offering for O365. There are some hybrid plans available for the products as well (when dealing with cost etc.)
I hope this helps you a bit, and it sounds like maybe a Nintex Partner could help you out and there is a lot of literature out there as well as to the different offering available.