Modern computers are very reliable, but they are not

Reliable machines still commit errors from faulty data or instructions, so they are never completely infallible.

Speed is something these machines genuinely possess, so fast does not fit here.

Great processing power is also a real trait, so powerful does not complete the sense.

Falling costs make many of them affordable, so cheap is not the quality they lack.

The whole point is that dependability does not mean perfection.

Add-on: most computer errors trace back to human mistakes in data or instructions, summed up as garbage in, garbage out.