In a nutshell, start with a 50+-year-old movement. Then, add today's slightly higher voltage 1.55v batteries and parts which don't really wear down but are old, and likely have been manipulated by repair persons in the past during previous repairs when they may not have been quite as good at their job as they should have been, and you have a timepiece which is challenging to begin with. Set it exactly as the manual suggests and it will likely be fine, although additional adjustments will be needed because of the higher voltage battery being used.
After a few adjustments, an Accutron can generally be made to operate as we hope. However, there are those, and I'd say 1 in 10, which can be stubborn and parts may have to be replaced in order to find a winning combination of pawl, fork, and coil assemblies. Even then, another part may rear its head which will have to be discovered with some trial and error. So, 1 in 10 Accutrons will need additional time to complete their repair. With the exception of the coils, most all of those parts are in ready supply so if such a part is discovered I normally just replace it at no additional charge and they almost always end up being a part which cannot easily be ascertained visually as being the culprit. In the end, though, I will always get an Accutron to operate as it should and take a lot of pride in the fact I never will give up on one. You will always get it back and timing as well as can be expected...with today's higher voltage batteries, I shoot for +/- 5 minutes per month, which is close to chronometer standards of +/- 3 minutes per month.
Today's batteries throw a bit of a curve into timing an accutron, but they will always perform well once serviced correctly if you can be patient with me on those which are stubborn. I still prefer to keep them away from water, aside from hand washing, since the parts are so much harder to find nowadays. I stand behind my work so be assured that if yours starts acting up after service, I'm always willing to get it back and readjust until I can get it right. They were so much easier to adjust back in the day of lower voltage but after sometimes a lot of effort, I'll get it an no additional cost to you...just time on my part. Those I'm adjusting, when they misbehave, get daily attention with many adjustments to indexing until I can find the exact setting spot to accommodate today's higher voltage. And as I said, if needed, I'll swap forks to find that winning combination. A little known fact, each fork was oftentimes modified individually years ago to achieve timing so with today's batteries it can be very difficult for anyone not as familiar with an Accutron as I am.
Also, about the only part that ever needs to be replaced in an Accutron are the coils. I would say I need to replace them in 1 out of 20, on average, for accutrons in for repair. Occasionally, something else is needed but I rarely charge for those parts as I have thousands of them.
There's no way to know when a coil will go bad. How many electric toasters have you seen working as they were meant to 50 years later? It's incredible...the livelihood of an accutron coil, the one electrical part in an Accutron still working after over 50 years. Sometimes, even after being serviced, a coil will go out and need replacement. I hate telling a person, especially after a service, that they have bad coils because that's the one pricey part in an Accutron and I do have to charge for them, but for our love of these incredible watches, it's usually worth the investment.