Commissions and custody fees seem ok. Interactive Brokers would probably be almost safer than holding the keys physically yourself in some cases (no loss or theft risk). Of course, the biggest risk to consider might be them freezing the account as with any third parties but IB is probably one of the safest exchange to hold your digital assets isn’t it?
The cons are i) you probably do not own the keys, you just buy the "paper" financial product similar to buying a gold ETF and ii) you can't send BTC or ETH to other third parties (for payment for instance).
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