From a safety perspective (ignoring cost, though which is also critical for viability), I mean minimizing the number of 'Jesus bolts' in the design - single components that if they fail, you are in a world of trouble. In the Lilium for example, what if the tilt mechanism for one of the main wings locks up while in cruise, and you only discover this while coming in for a vertical landing? This could be due any one of many possible reasons: SW bug, electrical failure, mechanical failure, debris jamming the mechanism, etc. You've got one wing producing forward thrust and the rest producing vertical thrust which is not likely to end well. You need to either prove that every part of this particular system is extremely reliable or you need a redundant backup system in place. Due to the design, neither of these options are easy here.

Obviously you need to assume there will be motor-out situations in any eVTOL design and the system needs to handle this with full payload over the flight envelope needed for a safe landing. Lilium has the advantage of many propulsion systems to take over the lost thrust, but many other eVTOLs are questionable here.

The same thinking has to be applied throughout the entire aircraft design (electrical and mechanical) and then you need to test, test, test. Each failure mode, using full payload over the full flight envelope, stability testing under differing weather conditions, the list goes on. To give you a rough idea, having a flying prototype is great but once you are comfortable enough with the design to put in a test pilot under controlled, monitored, and ideal conditions, you are maybe 20% of the way towards something you can responsibly put into service. If you want to see a simple design, check out the Opener Blackfly.

Is an airframe parachute essentially a requirement for a vehicle like this to be viable?

It certainly increases the safety factor, but should never be relied upon as the sole redundant backup system. There are situations where it will not do you any good such as hovering at low altitude. If the aircraft does not have a reasonable glide slope, most people would feel a lot more comfortable with one.

The Lillium aircraft has no glide slope at all. It has wings, but there are no control surfaces to maintain a glide in case of power loss.

More cost.

An airframe parachute is included in the Lilium design. At least it was last time I checked.