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It's more the fact that the thread on the metal one is 'too aggressive' and because it's going into plastic has over time cut across the threads and then caused them to detach themselves from the levers. Yes i was also doing the reverse until click before inserting but 🤷♂️
The nylon is rounded rather than pointed so won't be able to cut. Basically as @chiroshi says.How often? these were on their 3rd bleed technically. But the 3rd bleed took a few attempts - new calipers. So maybe 5 X inserts give or take.
But the bolt that caps the reservoir is metal. Just be careful and don't overtighten stuff.