I agree, their systems are terrible.
I set my previous car up for the direct debit road tax (6 monthly). it took it the first time, then after 6 months it did not come out automatically.
I watched for it, assuming it might just be delayed by a week or so, but nothing.
Then, I got a letter through the post, and it had a fine for not paying my road tax!
I called them up and complained, and they said that everything had been set up ok but for some reason THEIR SYSTEM did not request the direct debit to come out. they admitted it was their system's fault but said there was nothing they could do and had to fine me anyway.
I had to cancel the direct debit, set up a new direct debit (did it monthly this time so I could keep an eye on it), backdate my tax and pay the fine. very annoyed.
When I went to tax my new car, their online direct debit system wasn't working so I had to pay by card.
The whole system could be far far better, and simpler. but it probably never will be.