But still, an analogue instrument would still have a very hard time to break down, compared to a EADI
This was the case not too long ago. Its taken a long time, but these electronic instruments have finally achieved a reliability level that makes them suitable as a standby instrument but with MTBF rates that are still slightly lower than many mechanical instruments.
25 requires that the loss of all sources of altitude or heading or attitude or airspeed must be extremely improbable. That includes the standby and main instruments. The term "extremely improbable" relates to a failure rate of 1x10E-09 per flight hour. So in many cases, to bring this failure rate to an acceptable level while using a less reliable electronic instrument, the main instruments may have to be a more robust (expensive) type or they may be a requirement to raise the redundancy. ie: 3 main altimeters instead of 2.
Of course the electronic standby instrument must be paired with a power supply that is independant of the aircraft generating system. This is usually in the form of a battery pack that is charged, but isolated from the main aircraft electrical system.
The battery packs must be capable of supplying power to the standby instruments for times varying from 30 minutes to a few hours depending on the operational approval sought.
It is interesting to note that the standby instruments must become operational with no flight crew action following a failure of the electrical system.