Advantages to Flat site structure versus sub-sites?

[AlexH980]

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.

[#2]

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.

[#3 · Accepted solution]

@AlexH980 

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."

From Planning your SharePoint hub sites, also more guidance here, Plan the site collections you need to create.

[#4]

This makes a lot of sense! Thanks @Cian Allner and @Chris Webb 

[#5]

It APPEARS to actually just be the way Microsoft WANTS it to be. There is a little more flexibility if/when things change in a larger organization - but functionally there is little, if any, benefit.
This is the preferred architecture for SPO. It becomes obvious when you create subsites and see the bugs in the navigation header (Changes at the top level have problems being inherited), as well as bugginess in the settings screens for lists, libraries and the subsite itself. It's obvious the full, seamless function of subsites was neglected.
This is part of the vision and strategy to make everything homogenous with their cloud offerings. The main benefit being a narrower scope of what is supported, and so a faster evolution to the most efficient architecture over the life of the tenancy.