I'm reading this pretty nice summary of the relationship between wind/solar adoption and reliability. In short, no noticeable relationship thus far, and seems to be a lot of room to run (plus some straightforward policy ideas about how to extend it): ieefa.org/wp-content/upl… 
This paper is a nice review of actual practice in grids with a lot of wind/solar. What they are finding is not terribly surprising: If you account for minor policy/market changes and the indirect economic response from other generators, integration is easy for quite a while.
But a nice summary of the different challenges and actions taken in different places: Texas, California, Ireland, Spain (and a bunch of others).
Adding a bit of commentary: I wouldn't expect to see reliability impacted by higher RE because reliability is the most binding element of the cost-RE-reliability triangle. System operators will either stop RE increases or spend on infrastructure rather than lose reliability.
The authors here do understand that and prefer to use amount of curtailment as a better representation of integration challenges. The figure I showed above was just their first look at this relationship, with deeper analysis that follows.
