Just to declare an interest here, for those that don't know, I am Campagnolo head mechanic in the UK.
Lever failures are rare but can happen. Obviously, the older and more heavily used the lever is, the greater the risk of failure - wear and tear can strike but a typical life for a well fitted lever on a well-maintained gear system is 100,000 km plus. There are composite inner shift levers in the "Campy Test Lab" which have done 1,000,000 shifts and are still intact.
So, commercial over, here's the technical bit:
Repairability depends on model.
There are two basic types of Campagnolo lever - "Escape" or "ErgoPower" (up to 2009) and "PowerShift" or "UltraShift" (after 2009). To be strict, PowerShift actually appeared in 2011 and there is also some crossover between generations - Escape is actually still made in the Xenon range for sale to specific bike manufacturers.
Escape and Powershift can have new inner levers fitted but apart from that are basically not serviceable since the problems that they exhibit are all caused by damage to, or premature wear, in parts that are not available as a spare part - in these cases, "slipping shift" is generally cased by premature wear in the lever internal, frequently cased by the use of mineral-oil based lubricants (although this is not the sole cause).
It is strongly recommended that Escape levers are not lubricated - they are lubed for their initial service life at the factory (generally held to be 3 years) on assembly and if serviced by a Service Centre thereafter are lubricated again the same way, with the same, compatible, lubricants.
The new inner levers that can be fitted can be alloy rather than composite, so in cases where inner lever failure occurs (most frequently incompatible front mech, incompatible chain / crankset or poor set up - or all 3 - this is sometimes a good option.
ErgoPower and UltraShift are fully serviceable. In the case of ErgoPower, the springs and spring mount rings referred to in earlier posts are all freely available although many users prefer to send the levers to a Service Centre such as ourselves, as we have all the spare parts that might be needed along the way (sometimes the job is more than it appears at the outset). In the case of UltraShift, Campagnolo do not supply many of the individual spare parts but at Velotech, we do keep them all on hand and will service UltraShift levers for customers on request.
Finger lever failures are usually a result of excessive force being applied on a repeat basis. As mentioned, this can be an incompatible front mech (wrong pull ratio), or a poor choice of chain (typically KMC) or a poor choice of chainset. Other issues we see from time to time are also poorly assembled or routed cables, low quality, compressible outers, plastc ferrules (not in themselves a cause but often used in combination with poor quality outer because it makes for a cheap package).
One of the worst combinations is also often fitted, especially by bicycle manufacturers, again for reasons of cost - FSA chainset (lacks really good uplift ramps and pins and the rings are actually too far apart for a Campag 10s system) and KMC chain (the link shape doesn't "grab" the already sub-optimal upshift ramps and pins very well).
The end result is that the finger lever is held across too hard, for too long, in an attempt to "force" the change. This results in, after repeats of this over a period of time, a lever failure.
Even in cases where all the parts are Campagnolo and correctly compatible, we see many, many cases of poor set-up. Front mechs too high, too low, wrongly angled ... poor quality clips to hold a braze on mech to a frame with no braze on so that the mech can't be correctly positionned (or in some cases, it can but it isn't) ... incorrect cable tension ... attempts to use a double lever on a triple system - the list goes on.
One's own experience is often a poor indicator - but it's maybe worth saying that I have always used Campagnolo, since I was 16 (I am 54 now) and have raced every season ... since ErgoPower was first introduced, I have used it, in almost every case with the composite inner lever and never suffered a lever failure. My wife, who does at least as many km as me in training and racing and has done for the last 26 years, since we have been married, has also used Campagnolo all that time - she has never had a lever failure.