There are also limitations in aircraft POH, the DA40NG has electric ignition with FADEC engine which would be fun without a battery, the main battery may gives 30min and backup emergency battery about another 30min, if you plug backup battery it means land ASAP and will not fly again until an engineer sees the aircraft, same for fuel emergency transfer if you break-glass and cross-feed, you are grounded for next flight !
On aircraft types with “engine mags” the whole flight will be OCAS in good VMC, maybe in landing configuration on VFR MSA, otherwise one may need to switch his radio ON across borders or when jets are around 
for aircraft with electric fuel/flaps/gear, I expect any transition training to include discussion on fuel burn and landing without flaps/gear or manual/emergency extensions
The other problem is most GA maintenance facilities when you are flying away are inside CAS airports
Pilot_DAR wrote:
If the pilot understand the systems well…
Well, obviously in the cases where it isn’t safe then it isn’t safe. But that wasn’t the point. The point was that there is no blanket ban in Europe for flying with an inoperative system even if you have no MEL. I have no idea about North America.
All equipment was working at the time of departure.
The belt had come off and was damaged (not completely broken) when I landed at a fairly isolated island strip (2018). I removed the engine hood, cut the belt off completely and checked for visible damage. I called a fellow pilot who is also a mechanic and discussed the situation, and I concluded it was safe to go (without an alternator belt). The battery could run the avionics but not start the engine. After checking weather and NOTAMs once more, I hand propped and flew back home. My home field is controlled with ATC and I had planned with landing at nearby strip in case I ran out of battery for the VHF. However, when I explained my situation to my ATC 40 NM out, they gave me landing clearance right away, only asking that if I still had battery power when entering, they would like to hear from me.
With the precautions and good weather, I regard the risk as quite acceptable, almost the same as for normal flying. Hand propping was probably the riskiest part.
In a single with electronic ignition I think the risk assessment would be different. At my airfield a C172 and a PA28 have recently had one magneto replaced with electronic ignition. There is no battery backup in the system, so a loss of ship’s power kills the electronic ignition immediately, hanging the engine on the single magneto. With the prospect of likely alternator failure, possible loss of electrical power, and a remote but existing risk of failure of the single magneto – without power to squawk or make emergency calls (or run the flaps if electric) – then I would probably regard it as a no-go except perhaps for short hops if taking off with a fully charged battery.
Similar considerations would apply for e.g. Thielert diesels with FADEC, where the engine is 100% dependant on electric power but there is a dedicated backup battery for the ECU’s.
Peter wrote:
Assuming the drive belt is intact (if not, propeller has to come off to change it)
That’s the reason why I have a spare belt wrapped around the front part of the engine. If the belt fails, I have one more shot before the propeller needs to come off…
