Goal
The single-page complex form allows to fill in a large amount of mostly independent, but still connected data sets. While these data sets could be spread among multiple tabs, there's a clear advantage to make all of them visible at once on one page.
In order to improve orientation in the form, data shall also be grouped visually. Each group of data must be consistent and constructed such that it may largely stand on its own. This type of form distinguishes between important and less important data sets, the former being visible at all times while the latter may be hidden at first.
Concept
As stated in the goals, we distinguish between data sets, which contain values, which have to be entered every or at least most of time a user fills out the form, and sets which are changed less frequently. We use a combination of field sets to group important data sets and accordion-type tabs for hiding forms for less important data sets. Important sets must be visible immediately when opening the form and may not be hidden by the user.
In contrast, less important data sets are mostly hidden, but show a title naming their topic and a short overview of the values of their most significant fields. These overviews are an essential add-on of the complex form: since less important data sets are usually not touched when filling out the form, the overviews must present their default settings in a concise, readable, quickly understood short sentence or sequence of words. We want to avoid that the user has to open these sets only to check the values contained in them. The advantage of the single-page complex form to present all settings at once largely depends on the quality of the overviews.
Do not use such a form, if your form actually does contain sets, which are more like multiple, connected steps. In such a case, use a multi-step process. Also, if the overview over all values is less important, your form may be less overwhelming, if you break it up and make the parts accessible using tabs.