Frank,
The main advantage of the plastic clunk tank is weight savings. Generally, the plastic tank is half the weight of a tin tank. Also they are cheap, available and you can change them out quickly. The disadvantage is the capacity sizes are pretty big jumps and the dimensions are a little wider than we normally use for say a FOX 35/OS 40 FP and are 1/4" deeper. So you need to have room to get the tank centered. Yes, you can adjust the uniflow if you use a fixed uniflow tube but most successful setups tie the uniflow to the fuel pickup clunk so it follows the fuel in the tank and keep the run smooth. So if there is room in the fuse it's good, if you need a specific dimension or capacity not so good.
Best, DennisT