"With the new syncing solution, you must open the Excel app to receive new responses. Otherwise, they will not sync to the workbook."
This will be problematic for several use cases that depend on the interoperability of M365 products. One example: it's not uncommon to build a simple Power BI dashboard with a MS Form's sync'd spreadsheet file as a data source--with automatic refresh and row-level security, this allows users to see an up-to-date log or visualizations of their own responses without having access to other users' form submissions, as they would have if the output sheet was shared directly. The form syncing update means that either (A) some user will have to have the spreadsheet open at all times, eroding the value of automatic refresh, or (B) the dashboard will be inaccurate. This use case demonstrates the benefit of MS Forms + Power BI compatibility and often delivers value to organizations in lightweight, everyday workflows where heavier development of Power Apps or SharePoint Lists would be cumbersome or expensive.
For another example, Power Automate workflows that back-reference prior form responses (even if triggered by the Forms connector, as suggested in this article) will no longer be reliable, as they will only be able to access responses that arrived prior to the most recent time a user opened the spreadsheet (I am not aware of any MS Forms connector in Power Automate that references prior responses...). In many automated processes, by nature, users are not interacting directly with the spreadsheet, and having to do so would introduce a degree of unreliability.
Might Microsoft explore the possibility of giving users or admins an option for how data sync is performed (the "old" way (automatically) vs the new (only when viewed))? Alternatively, an action in Power Automate's Forms connector that could trigger a response sheet re-sync might present a viable workaround. I manage plenty of forms for which the new sync option will not pose a problem, and would happily keep it for those, but this change negatively impacts several workflows that reference MS Forms data automatically.