> Is there any guarantee that if this goes wrong (let's say ETF's are hacked and lose their BTC) then assets in other ETF's are safe?
They're as safe as any other share or ETF of a company that could be hacked or robbed/suffer a disaster of some kind, or indeed any regulated bank, broker, exchange etc. As the BTC securities would be a small part of the total value of the Organisations holding them - I would assume that they would be capable of making up of any losses to holders due to their screw up (as happened when Nickel was stolen from a warehouse that was used as backing for Nickel futures contracts).
However, any other external crypto nonsense going south would likely cause BTC to drop!