Have you seen the product's web page you linked to? Scroll down a bit. Look for a logo that says "Secured With BitGo". To the right there is text "What happens to my coins after I deposit them". There is a question mark next to that. Click on that question mark. It now reads:
"Coins may sometimes be moved in and out of exchanges and hedge funds as they are being lent out. In these cases, the coins over-collaterized with fiat."
It's not my opinion that the user doesn't have full control over the Celsius wallet. That's what the product page states as fact. My opinion is that if my crypto is moved in and out of my wallet then I want to be the only one doing it and I'd rather not take somebody's word that it's all backed by fiat.
I agree with @holovision.