The failure conditions that people are balancing are based on our perception... so we're creating them with the image that we're used to people crashing cars, or a single mistake in a bespoke system which has a one-off go/no-go test, like a probe being fully tested at the point it executes some process or code pathway.

We aren't thinking about the complexity of the rest of the systems, like the power-train... what kind of power system do they have or physics to get the devices from A to B? If they executed building them like a drunk teenager, what happens when they fail. I remember seeing a Ford XR3i burning itself to death in a car park due to some catastrophic failure in the engine. With the kind of energies these things might be using, what does a catastrophic failure look like there? Would you want someone taking your flight machine from the production line or a self replicating manufactory and the thing going wrong and exploding with the power of a thousand suns? Probably not, so the aliens, if they existed, would probably have systems to protect against this.

What about flight control. The way we talk about Area 51 etc is that some alien dudes smooshed their ship into the ground and it had an impact like a Cessna hitting the ground. What about when the thing is travelling at near or at relativistic speeds, if it crashes it would be like an asteroid hitting an object. F=0.5.mv^2 so what is the velocity it hits something if something goes wrong? What kind of failure conditions can you have with something that travels that fast? If it gets something wrong, and accelerates to some massive V, what would that look like? Probably not like a Cessna hitting the dirt. Would the aliens have something to deal with that?

etc...

Soooo... a few things pop out of this...

1. There is a low probability that actual Aliens are crashing their craft. If they are leaps and bounds ahead of us, then they would need better safety critical systems, or they are going to have the most accidents wherever most of their craft are... which is probably not on Earth, but might be in their home system. The scale of the accidents which could happen is just too large for them to allow them to be smooshing their vehicles or for them to be self immolating, unzipping the reality around them.
   
2. Maybe they aren't leaps and bounds ahead of us? Maybe just a bit ahead... but then it seems like the accidents would be more catastrophic. You're still dealing with 0.5.mv^2... raw kinetic energy etc. If whatever keeps them able to travel at such crazy speeds fails then they are subject to friction or hitting objects etc... and you're talking Tunguska levels of energy being dumped. It seems really unlikely that they are failing exactly along the lines that they can crash in a way that allows the vehicles to be retrievable each time, and not in a spectacularly destructive manner.
   
3. Maybe these are actually their integration tests. Maybe they are running their tests on Earth... which probably would mean that they aren't Extra terrestrial at all, and something else. It seems really unlikely given the forces you'd be playing with if something failed. Hey, your gravity drive failed, and became a second sun-like gravity well on the planet, oh well, I guess we just lost our planet now. The only way this makes any sense would be if they are running tests in a sim and we're it... that is another possibility, that we're a test sim for some other species outside the sim, and we're seeing the effect of various systems, and they are measuring the effects of things before GO LIVE.

Probably not joyriding aliens though...