I installed this distributor because like most owners got fed up with the high cost and frequent replacement of the vac advance module.
It was a factory approved replacement for years and sold as a kit so people who ran cars prior to 1979 could enjoy an electronic distributor. Expensive, even then, but nice to have especially when compared to points.
Some misconceptions about the vacuum advance persist. I suggest anyone curious as to how this thing work get in a fuel injected Spider 2000 that has everything set to stock (timing, etc.) and the vacuum advance is known to be working. From a dead stop drive the car. Then, open the hood and disconnect the advance from the manifold. Now drive the car. Note your 0-30 MPH time goes down by about 1/4. Same "operation" can be tested on any car with the vac advance but the FI is most noticable because L-Jet is (a) so smooth and (b) the most potent of the stock fuel delivery systems.
I've found it to be a reliable device that results in smooth acceleration on every TC motor I've installed it on. Any of the ADF carbs and most of the DMS/DHS/DFE series have or can be easily drilled to have the correct port to drive it. Some of the IDFs even have them, I think the 70 series but it's been too long to really remember.
As far as the unreliable comments, yes, a 30+ year old paper and rubber diaphragm is likely a failure by now. Replace it and remember that exhaust manifold heat shield you threw away or your car never had? It helps protect the diaphragm.