The primary reason to build or not build a WCD versus a “traditional” skid-steer drivetrain is what your resources are. Some teams resources lend themselves to a more traditional drivetrain, some towards a West Coast Drive. It’s worth noting that time is most definitely a resources. Probably the most important one. Just because you have the fabrication capability and money to make something doesn’t mean it’s worth time investment during the season if it’s not an efficient use of your time.
The primary disadvantage of a WCD over a more “traditional” drivetrain is that it usually requires more precision machining (yes there are COTS options like the VersaBlocks that can let you build one with hand tools). WCD requires live axles and does not offer as many simple solutions for chain tensioning. Dead axles tend to be a little more forgiving, but nothing that can’t be achieved with time and attention to detail. Bumper mounts also takes more thought.
The primary advantage is typically maintenance and access, especially with replacing wheels and worn tread. There are some marginal performance benefits that can be gained by putting your wheels a little farther outboard than usual.
Sheetmetal construction methods are also generally not well suited to making WCD either, which is why you see many teams with sheetmetal sponsors opting for other drivetrain styles.
It’s worth mentioning that many of the challenges associated with building an effective WCD are obsolete due to new COTS options and the proliferation of exact C-C belts and chains. Machining the bearing blocks, developing a good chain tensioning method, wheel retention, ability to make custom wheels with hex bores and building a transmission with the correct output speeds on a live axle shaft are all aspects of WCD that are now available as COTS or able to be fabricated without a mill/lathe. These weren’t available even a few years ago, which made WCD out of reach or not worth the resource investment for many teams. At the same time, it also means that many of the advantages of a WCD are now available to “traditional” drivetrains as well, so the lines are blurred a bit.
From a COTS standpoint, if you’re not doing any precision fabrication yourself it’s still likely cheaper to take your KOP chassis in the kit or purchase the VexPro 2014 Drive-in-a-day (both of which are “traditional” drivetrains) rather than spend money on the various bearing blocks, bearings, etc for a WCD.
Don’t make drivetrain decisions in a vacuum, especially if your team already has a particular drive style that you like using. I’m not saying don’t switch, but be sure you’re considering what the cost of switching would be (time, money, learning curve, etc) and what problems you’re solving by making a change. Is it cheaper? Maintenance? Better performance? Fewer or easier to make parts? Then figure out how switching to a different drivetrain style addresses that.