Comparing controllers and pilots is like comparing apples and oranges. The required skill sets are quite different, but perhaps more importantly in ATC there are not as many chances as there are on the pilot side. If you don't get an ATC job you can't just keep your licence current and look around for other ATC jobs!
I was on one of those courses mentioned by Lon More where only 5 made it through the college (sadly I was one of the 7 who didn't make it in my course). Halfway through the course we were 8 students left. I also have a CPL and am now doing the ME/IR (all state-sponsored, given to 25 students each year, so there is competition, similar to the ATC selections). All 25 of us are still here, and I expect all of us to pass the CAA check flight as well. If the training and exam philosophy in pilot training would be similar to ATC training though, I suspect we would have a much higher failure rate. In my experience, in pilot training it's recognized that nobody performs perfectly all of the time, maybe not even on the check flight, but one mistake here or there doesn't equal failure, unless it's a major screw-up. Even if it is, the pilot can always fly the check flight again.
Don't forget, when a trainee fails it is normally a 100% loss of all the investment made in him, although it is sometimes possible that he finds employment in another part of the organisation.There is no possibility of going back and trying again, even with another provider as they will be looking into the student's background. Failing the ATPL along the way still would leave the student with possibly CPL/twin/IR ratings so still with a (limited) job market and the possibility to re-sit the ATPL at a later date.
In ATC you are continually in training until you validate. If you don't make it to validation at your unit for whatever reason, it's normally very difficult to find somewhere else to finish your training and start your ATC career.
For pilots, I'd say the worry is not the training or "failing the ATPL", but finding your first job. The "student" with CPL/IR/ME is not really a student but a fully licensed pilot, but will possibly find it just as hard to find a job as it is for the not-yet-validated ATC dropout.
To make a (poor) ATC analogy of it, it would be like training for TWR at the college, and then sending your CV to every airport, maybe working as an AFISO and going in the TWR sim once a year to keep your ADV rating current. Then you realise that an ADV rating on its own will get you nowhere, so you buy an ADI rating. Then if you're lucky enough to get that TWR job, with a few years experience you might have a chance on an APP job, but of course you would have to pay for the APS rating yourself...