Yes Arduino are reliable. (At home, I have some project that works for several year without any issues)
But automotives environnements are harsh (vibration, temperature, etc) and OEM car electronic have much higher reliability standard than arduino. (especially if we compare Chinese clone with "so-so" manufacturing quality)
According to Murphy's law: "Anything that can go wrong will go wrong"
Even if it is unlikely You can have hardware failure (Hand made PCB are not very reliable, bad connection, blown fuse etc)
If (or when..) the arduino fails you will end up with nothing displayed on the cluster, it is not the same thing as loosing only speed limit indicator (or the car radio)
For me it just a risk that need to be taken into account. And I wanted to highlight it.
Every body can decide if the risk is worth it or not.
In my personal opinion, it is not worth it, but other may have different opinion
Having android auto vs risk of arduino failure and lossing only car radio-->I go for it. (big benefit low disadvantage)
Having speed limit display vs risk of arduino failure and loosing cluster --> I don't go (small benefit big disadvantage)
But if speed limit with single shield is possible, I go for it (And
@RuLe seems to say it is possible
)