Are they any broader functional advantages to setting up a flat SharePoint site structure, instead of sub sites? While I understand that nearly all sites are sub-sites, the majority of advice I've read recommends implementing a flat SharePoint structure. I'm just not clear on why that's the broader preference.
1. Hub sites and child hub sites have to be Site collections. 2. All development is going to hub sites / site collection level not so much subsites. 3. Office 365 groups (Team site, Teams, etc.) are all root site collections
Additional things like site sharing permissions and per site sharing defaults, Guest access, etc. work at the site collection level. Admin center shows site collections, just everything is built around site collections now, it's the best way to go about it because of it. shorter URL's for Sync. I'm sure there are other things I'm missing, but Microsoft has said themselves everything is being designed around site collections and not sub sites structures.
This was the key selling point for me, the ability to manage and support the organisation better, rather than a hierarchy that will be inflexible and will only add challenges as needs evolve:
"One of the key principles of modern intranets based on SharePoint is that each unit of work should get a separate site collection to optimally manage governance and growth over time."
"In the past, many organizations used subsites to create connective tissue for their intranets, using the site collection's shared navigation to connect sites and the hierarchical structure of subsite relationships to nest sites within sites. However, subsites don't give any room for flexibility and change. Since subsites are a physical construct reflected in the URL for content, if you reorganize your business relationships, you will break all the intranet relationships in your content. Subsites can also create challenges when it comes to governance because many features (including policy features like retention and classification) in SharePoint apply to all sites within the site collection, whether you want them to or not. This means that you must frequently enable a feature for the entire site collection, even if it's only applicable to one subsite."