Very similar experience, I often open the app and it is still hung up on my previous (completed) order and I have to clear out the cart. And like you said, the "offers" are good (actually McD has raised prices quite a bit, now with the offers I'm paying about the same as face value was only a few years ago).
I will also say, despite the issues, it's better than most competitors. BK and Wendy's apps are both full of problems as well. I have had multiple accepted and paid orders on the Burger King app, when I got to the store they didn't have the item, weren't accepting mobile orders, or were totally closed.
After you order and drive away, I’ll find the app hours later still tracking me and using GPS. I almost always have to force close the app.
Then the next time I use it the previous order won’t have cleared out despite me picking it up, and I’ll have to go in and “cancel” the order.
The coupons are good though. For a week or two it was giving 30% off, and almost always has a 20% off coupon.