Here's the deal for me.
Since I've used permethrin, spraying the outside of my pack and the outside of my down quilt/sleeping bag, I have never had a bite when using my sleeping bag.
I dare say there are PLENTY of pilgrims who can say the same.
Personally, in all these years, I haven't heard more than one or two people say, "I sprayed my stuff and STILL I was bitten."
And a person could argue they didn't apply the permethrin correctly.
Whether some study says it does or does not work doesn't matter to me.
That's like an argument about are eggs, milk, meat good for you or bad for you.
Conclusions to scientific papers, unfortunately, all depend on WHO is paying for the study.
Regarding safety, I have Multiple Chemical Sensitivities, so the choice to use permethrin was a serious one for me.
I'm the canary in the coalmine.
If I react to something, you'd better think twice about using it.
I do NOT react to permethrin, which tells ME it's most likely safe, if only used for a few hours each night for a few weeks, and if it's dry.
You can BET the majority of the albergues spray some type of poison on a regular basis, so it's a bit of a moot point to worry about sleeping around chemicals. The beds, floor, walls, and air are getting doused regularly!
Regarding effectiveness, the proof is in the pudding, imo.
Only one of my client has been bitten - and she did not spray her stuff.
And regarding those who say, "I never saw a bedbug," well, you're lucky
It was the luck of the draw.
MOST albergues do NOT have ongoing bedbug infestations because they regularly spray and are vigilant about cleanliness and watchfulness.
But all it takes is one pilgrim to bring them in, and a bed is infested.
So do whatever you're comfortable with and don't worry yourself sick about it.
If you're afraid of chemicals, then don't use them. Learn to spot the sign and be vigilant.
The best idea I heard was encasing your pack in a plastic bag at night (for protecting your pack).
Unfortunately, this will do nothing to protect your body at night while you're sleeping, which is when the bugs feed.
If you aren't afraid of chemicals, then Permethrin on your pack and bag, or DEET on your body, has proven in the past to work.
Buen Camino!