I'm assuming we're talking about the VW Bus and VW Vanagon Westfalia conversions, not the Eurovan ones. I don't know anything about Eurovans.
I've owned a 1975 Bus, a 1981 Vanagon, and now a 1990 E-250 Sportsmobile.
VW plusses:
- Incredible space efficiency, thanks to the forward-control, rear-engine design. At 15 feet long, probably the shortest vehicle that can sleep four.
- Easy walk-through from front to rear -- the front floor is flat, no "doghouse."
- Independent suspension. Vanagons also have rack-and-pinion steering and are the best-handling vans I've ever driven. Very nimble once you get used to the body roll (which feels accentuated by the driving position over the front axle.)
- Relatively lightweight. A Vanagon Westfalia conversion, loaded, is under 5,000 pounds. A Bus conversion, loaded, can be under 4,000.
- The interiors are quite well constructed, although as with any old RV the refrigerators tend to lose effectiveness over the years.
VW minuses:
- The engines are all terrible, and badly let down otherwise good chassis designs.
- The air-cooled engines in buses are maintenance-intensive. Vans with the Type I engine are badly underpowered, later ones with the Type IV engine are better and more durable. The heating system on air-cooled engines is weak unless you get a gasoline heater.
- Diesels are horrendously underpowered (49 horsepower!), and noisy.
- The water-cooled boxer engines suffer various corrosion and head gasket issues. The complex electronic fuel injection systems haven't aged particularly well.
- There are engine conversions, most commonly using Subaru engines, but these are expensive. The Subaru ones also reduce ground clearance quite a lot.
- Relatively few campers were equipped with A/C, and the A/C systems in the earlier ones were ineffective.
- It's hard to find tires with a high enough load range that fit a 14-inch rim. It's possible to use 15-inch Mercedes wheels if the center hole is re-machined to fit the VW hubs.
- They're all getting long in the tooth. The newest US-market Vanagon is now 28 years old. Some parts are getting scarce and there aren't a lot of mechanics with experience in many areas.