I'm reading the article on Service Fabric cluster capacity, reliability tiers, and durability tiers. The article (at this time of writing) lists the following tiers:
Is there any disadvantage / performance implications to using a high reliability tier (like Platinum)? I'm creating several clusters that will span at least 50-100VMs with a combination of stateless and stateful services.
Not really. Yes, technically the replication of changes to the system services will take longer, however in practice this has not made a significant difference in the performance of the system services. The replicas are almost always in a set of ~5 physical racks, the data is stored on local disks, so the performance tradeoff is (usually) more than worth the safety gained. YMMV of course.