The double retirement situation at Spa was extremely disappointing but not IMO "unacceptable". This strong talk is not accurate or useful to describe the situation. Neither engine let go in a big way. MB PUs were going boom around the track. This was Sainz's first mechanical DNF since his Renault PU blew in the first GP in Australia. The background of Renault reliability is the unprecedented reliability of PUs this year in general. They are the least reliable PU but not shockingly so.
After the destructive relationship with Honda it should be obvious to McLaren fans that this relationship needs to work. There's no other option, at least until a new manufacturer comes into the sport. Renault are making progress, not as much as we would like, but there is near parity with Mercedes. Ferrari is a little further up the track.
Personally, I'm more concerned that the team failed to get the MCL34 in the zone during FP1, 2 and 3 and entered qualifying guessing. Why did the team fail to make ANY progress through three sessions of practice? It was probably more by good luck than execution that they halved the gap to the front of the F1.5 runners for Q1 and 2. These are more troubling issues and need to be sorted. To be fair this has not occurred since China way back.
Why did Sainz get sent out on used softs in Q1? Fail! Maybe they lacked the confidence they had sorted the setup as seen from the dismal practise times, but you have to put your best foot forward when your back's against the wall!
The only good news were (1) Lando's great start (MCL traction wins again) and (2) his stunning race pace and tyre life. Fair to say Lando has had cruel luck this season and is now due a trouble free run through to Abu Dhabi.27% voted Lando driver of the race.
Spa appears to be tire temp related, they apparently did improve the car on the Saturday, but the team thinks it was the drop in ambient temps at the track on Sunday which made a lot of the difference.