Before you consider the system to be unreliable and inefficient, take a moment to think about what's happening inside an engine. You have a metal piston, sliding against a metal cylinder wall, thousands of times per minute, at ~60-90 miles per hour. And none of the comments seem worried about this? Rightfully, because cylinder walls use oil spray to minimize the friction and reduce the wear. In this clutchpack, we're talking about different materials, designed for the application, bathed in oil, with a massive surface area thanks to many clutch discs (spreads out the friction, minimizing wear). 0.7% slip is extremely low, and the friction created causes a torque to be sent to the rear wheels, thus most of the energy still goes towards powering the vehicle, with a tiny amount lost as heat.
Still not convinced? Prior to this YouTube thing, I worked in the forklift industry. Some forklifts use the transmission to brake, rather than wheel brakes. These are wet, multi-plate clutch packs, exactly like you see here, and they do nearly all the braking for a 4-5 ton forklift. That's real heat, because brakes (in this case a clutch) are designed to turn kinetic energy (the forklift's movement) into heat. These clutches can last the life of the truck, thousands and thousands of hours. It's very cool, and pretty incredible! Not to mention, pretty much every multi-speed automatic transmission is using clutches. Heck, a single-disc, dry clutch inside a manual transmission, can also last the life of the car! 100,000 miles no problem, if you're not abusing it.
My only point here is, before you assume it will fail, why not wait and see how it turns out? Toyota has a reputation for cranking out reliability. Why assume they haven't done the testing and validation this time? If there's info on GR Yaris burning up these clutch packs (it's been out for a bit now) - definitely let me know, and I'm happy to share information on it! I'll remain skeptical that the clutch pack is the big reliability concern for this vehicle.