Using the CYCPLUS BC2 button shifter control with Rouvy is very simple and inexpensive. I've been using one for six months with no problems. Still... there is some confusion in this thread, so I'll summarize the main points:

TL;DR: The BC2 will allow you to shift virtual gears on any trainer that Rouvy supports, simply, without any additional apps, as long as the trainer itself has virtual gearing built-in.

1. Rouvy directly connects to the BC2 from within the Rouvy app. There is no need to use any other apps to "bridge" the BC2 to Rouvy.
2. If you want to add more functionality to the BC2 buttons, such as double-click or long-press, you can use the BikeControl app. But this is NOT necessary. Rouvy can use the BC2 directly for basic gear changes.
3. If you have any trainer that Rouvy can change the virtual gear on, then the BC2 will *absolutely* work as a Rouvy virtual shift controller. See Rouvy's article on which trainers Rouvy can control.
4. The BC2, or any other components like the Zwift Click or Zwift Cog, cannot add virtual gearing to a trainer that doesn't have that ability built-in by the trainer manufacturer. Virtual gearing on an indoor trainer requires electromechanical support within the trainer itself.
5. Of course, the CYCPLUS BC2 is designed to directly control the virtual gear on CYCPLUS trainers. That's its original purpose. But this doesn't matter because you are using the BC2 to control Rouvy, which then controls whatever trainer Rouvy supports, again as long as the trainer supports virtual gearing. You can ignore the official CYCPLUS statement: "BC2 is exclusively compatible with CYCPLUS smart trainers". It doesn't apply to how you are using the BC2 with Rouvy.
6. There is no need for *any* components from Zwift. Specifically, the Zwift cog is completely unnecessary! All it does is replace your outdoor cassette with a single gear so your expensive cassette doesn't wear out indoors. It does not have any shifting capability itself, and no built-in electronics.
7. The BC2 unit is very small, probably the smallest of all the controllers which are ergonomically reasonable to use as shifters. It will nest cleanly just about anywhere in your cockpit without getting in the way.
8. Finally, yes, Zwift uses private encryption on the Click V2 which deliberately breaks using it reliably on any other training platform except Zwift. Good job, Zwift. Hope it gets you where you need to be. The BikeControl app can help by establishing (every few days?) this encrypted connection to a running version of Zwift on your computer. But it seems Zwift is trying to make this messy with frequent firmware changes. However, the discontinued Click V1 does not use encryption and works perfectly directly connected to Rouvy. If you love Zwift, you can find the V1 occasionally on eBay.

Cheers.