I think we're starting from different premises - and perhaps different sources. As far as premises go, I'm including losses due to technical issues (e.g. engine failures, environmental conditions, faulty landings) because I figure 'toughness' includes more than combat survivability. I reckon if you can't keep your aircraft in the air it's a bit of a stretch to call them 'tough'.
On that basis, of the 25 R-planes actually flying combat missions I have 17 of them going down. I have the same three combat losses as you, but have another on 21 April crashing 'after combat'. For the Mourumetz I have a total production figure of 73 with 'about half' used in combat. If we err on the conservative side and put that at 33, only 4 of those were lost - 2 to enemy action, 1 to sabotage, and 1 to control loss. Granted they were working in a less threat rich environment, but as the R-plane figures (pulled from Rohrbach) seem to show, it wasn't external threat that was the problem.
In any event, this is all grognard stuff and shouldn't divert you from your grand purpose. All kudos to the team for the progress you've made, it's looking really good.