Invisible aligners with crowns. Any suggestions?

I have crowns on both my top and bottom teeth closer to the back (wisdom teeth gone). SmileDirect told me I was a good candidate for their invisible liners but I should know that my "crowns may be loosened" during the process. I had the hardest time trying to figure out why they would say that, but now I'm wondering if it's due to the aligners being snug while in and then pulling the aligners out to eat each day. Is it the pulling they are worried about and should I be worried about that?

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