The biggest risk in custodial investment is the bold part above which I only hinted at earlier. It is very easy to see them go corrupt in order to maximize their profitability. They can take people's money and tell them they have bought bitcoin for these people but in reality they never do.
This is somewhat similar to the problem we already have with centralized exchanges as well, many of them are insolvent and are running a form of "fractional-reserve" where they can't cover all the withdrawals if everyone wanted to pull their funds out.
The collapse of FTX in 2022 is the clearest example of the risks of holding bitcoin or buying bitcoin through centralized platforms. If I remember correctly, it was after rumors of a shaky financial relationship between FTX and Alameda Research spread. At that time, investors withdrew money en masse, but due to insufficient liquidity and the numbers displayed on customer accounts were just virtual. FTX had to suspend all withdrawals, many investors were unable to withdraw their funds and then as we all know, they declared bankruptcy and many investors lost their money because of the collapse of FTX.
https://cointelegraph.com/news/ftx-us-suspends-withdrawals-according-to-on-chain-data