In general if you've tuned a street engine that close to the edge you need to back off. In over 15 years of professional tuning I've learned that a good tuner is measured not only by performance but by reliability. A single sensor failure should not mean instant engine death.

Having tight limits on the sensor outputs and the fuel trims can allow you to detect the failure early. It's also worthwhile to study the failure mode of the sensor and make sure your strategy is compatible with it. In other words default to your safest timing and fuel combination and only switch when you're certain that the fuel sensed correctly.

We have flex fuel cars using only lambda sensor to deal with transients for many years and didn´t hear so much about broken engines...

other thing is split pistons, what dos it mean ?

1. if they are melted = it´s high EGT (lean condition)
2. if the piston groove were rings are mounted are broken = it´s generally too much detonation

(sorry if I misundestood what split pistons are)

Or they are of crappy alloy and it is an engine problem and nothing to do with ethanol or gasoline.

I've seen a few cracked pistons on earlier 1.9 PD engines on stock software for example. Nothing to do with the software or anything else for the matter, just bad casting/alloy.

The best way to know is examine the bearings and the cylinders.

Since emissions standards have increased in past years the manufacturers have changed the alloy for their pistons to allow for less thermal expansion and therefore tighter clearances. The addition of more silica means that they tend to be a bit brittle. This is why you frequently find broken ring lands in an OE engine but but bent ones in a forged one. Did they have a bad batch of pistons? Maybe.