Haha, don't give an inch to their narrative. I wouldn't give the fudsters any encouragement by saying "they share some properties with IOUs". Because you're not sending anything worthless - IOUs in Lightning.
If a discussion should have any value and not be only a game of propaganda and counter-propaganda, one should not try to hide disadvantages and tradeoffs.
The main property that LN channel balances are sharing with IOUs is obviously that it is possible that one party runs away with the money of the other one (if the other isn't watching his/her channel and settling), because of a disagreement about "how much one owes to the other".
Irrelevant. But that property also shares it with real Bitcoins sent to a special multi-sig wallet, that was confirmed on-chain, to fund the channel.