Well, an IOU of for example 1 BTC will likely be worth a value between 0 BTC (issuer = defaulted/fraudster...) and 1 BTC (100% trustworthy issuer). Values above that are of course possible but unlikely.

Your argument is now that it is hard to determine a proper amount to value this 1 BTC IOU. This is true however for any IOU and has been this way for millennia. Also to crush your dreams maybe a bit: It is my belief that 1 BTC (on the blockchain, as an asset) has exactly 0 value in e.g. USD as they are not redeemable for anything. This means anything that people pay for BTC is not based on their inherent value but rather upon their usefulness as money and store of value.

Your argument turned around would be the following: The inherent flaw of digital assets like XRP or BTC is that they cannot be priced by the owner, as they are worthless by themselves and it is not clear how useful they are to the world. So unless you can determine from everyone in the world how useful they find digital assets, you have no way to buy/sell BTC at their true price.

The solution to both your arguments about rating trust in IOUs and the opposite argument about uncertain pricing of assets is also known for millennia: an open market where people come up with prices and hopefully also match these from time to time based on both their needs/beliefs and other market participants.