The answer to your actual question cannot be offered as a general answer. I assure you that each person who tries to use assets and fails, is an individual case with it's own unique reason. Only another case with the same circumstances would be denied for the same reason. Instead of focusing on possible failure, I suggest you focus on why you will succeed.
If you are retired, have Social Security income, and have a large enough IRA to support yourself in the USA, then the person making the public charge decision can see that. It doesn't have to be this way, but if you can show your IRA is growing, even though you are making withdrawals to support yourself, that's another positive. But really, a Consular Officer is trained to understand whether the assets are enough to support you and the immigrant or not. That's what IRA's are for, to support retired people through their retirement years. It's no rocket science. If you know you IRA is sufficient, they'll be able to see that too.
That said, the people I've seen having issues with assets only, are those whose asset totals are either marginally qualifying or/and they are trying to use assets not considered "liquid". An IRA for a retired person, is definitely liquid. The question is whether it's enough. You already know if it is or isn't.