Never really had any problems.

Do yours freak out when you put your re-usable bags in the bagging area even when you tell it you're using 4 of your own bags?

no weight sensors, and where there are, you can put the bags on the platform, wait for like 20 seconds for it to recognize them and continue with the scanning.

Or have two barcodes on packages and if the wrong one scans (either because you aren't sure which needs to be scanned or because they're next to each other and you don't get a gun), you need a cashier to override?

never had that happen but this is the store's problem and I'm guessing doesn't happen all that often - neither the fault of the machine nor the customer.

Or have weight sensors that are just wrong about how much items should weigh?

never experienced that either with the ones that do have weight sensors.

Or only have enough room for like 2 bags of groceries but it isn't ok to take any out of the bagging area?

the ones with weight sensors have like a 1m*1m platform, there's plenty of room for like 3-4 full bags.