Reliability Of EVs falls into 5 main categories:
Quality over time
Durability deals with the ability of the vehicle to basically resist breaking down. Aside from designing a body and motor and gears that should be good for a million miles and batteries good for 350,000-500,000 miles, Tesla EVs have a distinct advantage over Gas Vehicles from a durability standpoint in that
there aren't over 1,000 spark plug induced explosions going on in the vehicle every minute (so less strain and heat on components that are NOT rated for 1 million miles)
the drive train and motor of an EV has roughly 20 moving parts compared to 2,000 in a Gas vehicle, so much less to wear out.
EV cars have less components overall than a gas car...roughly 25-33 percent less. So less components to break
Availability is typically just time to repair and how much that impacts your schedule. Tesla actually will send service cars to your location to do repairs whenever possible, including even rotating tires, helping with flats, etc. They also have service centers that you can take the car to, or they will tow it to if needed. As with any new car model year, sometimes availability of parts for accident damage repair can take longer. And for things like tires, alignment, brakes, suspension, cabin filters, body work, and even some battery service can be handled at your normal tire stores or service centers.
Dependability deals with a cars' ability to start when required and stay running. Starting problems on an EV are an extremely rare thing…akin to starting your iPad…and yes, you can reboot your car :). With no engine to start, no starter, no fuel to go bad, no rough idle, no scheduled maintenance (fewer hands messing with the innards of your car), no tuneups…there are fewer things to go wrong in an EV. Ditto for things to go wrong while driving, like they inevitably do in a gas car's hot engine cavity (hot radiator water to deal with, battery dealing with all that engine heat.) Of course there are all the parts that have to be perfectly running in unison like plugs, timing belts, fan belts, pulleys, o2 sensors, MAF, timing sensors, gaskets not leaking, burning oil, leaking vacuum hoses…long list.
Probability is just what it says…probability that you will have an issue with the vehicle. This is where small issues come into play. For example, Tesla had a lot of panel alignment and paint issues in the first year or so of each model they produced…which they corrected when asked, but that still is an annoyance for customers at the time. Furthermore, EVs are very much like tablets on wheels (very software centric) and a new experience in driving for most people. This causes more early on service calls asking how to get things like Wi-Fi and Bluetooth set up correctly (or even just asking what blue tooth and Wi-Fi is :)). The first time I test-drove a Tesla we got in the car and realized that, yes, we had a key card, knew where to set it, but didn't know how to start the car (foot on brake). So, anything new like an EV is going to involve more questions from owners, sometimes thinking things were broken when they weren't.
Quality Over Time
Quality over time is easy. If the seats wear out, stain easily, are a pain to maintain (so aren't) then this will ding reliability. From experience with gas cars, brakes wearing out quickly, knobs breaking off in cold weather, or headliner or sound insulation falling apart, all annoyances that impact your feeling of quality. And then, of course, are the normal engine related goofiness (rough or noisy idle, squeals, backfires, misfiring cylinders, acceleration lag, etc…we've all been there.) Luckily, none of these issues exist with the current generation of EVs, but only Tesla has had much longevity in the battery-only EV market, so time will tell now that production is in full swing.
Taken all together, reliability to me is simply whether I get frustrated with a car or not. If the car leaves me stranded in the middle of nowhere, that takes the car to the bottom of my reliability list. If something happens out of the ordinary and the manufacturer can't fix it because they can't reproduce it, ditto, way down on the list it goes. With my Tesla, I did have one odd thing happen soon after we bought the car. Out in the middle of nowhere (North Georgia), I noticed that the AC was too cold on my feet and that I couldn't enable dog mode…then the issue immediately went away. While sitting in the car, I put a problem report into Tesla and within minutes had a text message asking for the time the problem happened. After they pulled the logs, they told me it was likely a temp/humidity sensor behind the rearview mirror failing. They said they would send someone out from Knoxville to fix it in a few days after the part arrived. I decided that would be a bit of an extreme drive for them as the issue had gone away, so rescheduled for service to come out and fix it the next week when we were back in Florida. A 20-minute repair in my driveway and no issues since. Now, 30k miles with no other issues and more importantly, no trips to a Tesla service center since the first time I picked up the car keeps the car high on my reliability index.
The Gas Car Electric Component Surprise
I recently ran across a picture of most of the electric components in a typical modern gas car. After quickly crossing off the ones that an EV didn't need, I was a bit surprised at the quantity of even electric components that a gas car has that can wear out or need replacing. I knew my Tesla had no fuses or relays to wiggle or replace when debugging issues in gas cars, but this was an eye opener.
Then, the other day I watched a guy putting groceries in his Ford F150 at a Walmart and saw him finish, stand there a few seconds fiddling with something, then stand there even longer while the tailgate slowly closed, then he reached over and wiggled the tailgate. Had two thoughts: 1) really?!? (i grew up on a farm), and 2) I'm sure the diagram above missed a lot of electrical components on modern vehicles that I didnt even know existed.