It works for some kind of projects and i was motiviated for this talk, because I saw so many projects using manyrepos in cases where it may not fit very well.
A few points got me concerned in the slides - namely, concerning emphasizing how easy this makes to change things. I would consider that it is not a positive thing that adding a library is cheap and easy, and I find that component versioning can be bypassed even more so.
It all sounds like grievous local greedy optimization without a view of the entire lifecycle of the codebase. The slides say change is the only constant. I would claim that unless the change is backed by good and strong architecture - which throwing all architectural firewalls away does not support - then the end result will be a mess of spaghetti the older the codebase gets.
But, I may just be too much set in my ways of coding and I respect the fact that these may be non-issues in the particular domain these slides address.