CampaignNow was originally built specifically for NationBuilder customers, but it is definitely possible to use CampaignNow without NationBuilder.
You can create your direct email campaigns through CampaignNow and host those actions on a non-NationBuilder website (our forms work with any website platform).
You can then periodically export your data to a csv or excel spreadsheet so you have all your supporter information in one place (rather than connecting a NationBuilder account).
We already have a small number of active customers who use this setup. In the future we are planning to integrate with other CRMs directly, but we only support full NationBuilder integration at this stage. Organisations using other CRMs can export their data from CampaignNow in order to use it with their platform of choice.