I would like to know what am I losing by selecting a different manufacturer.
Lee filters are highly regarded for multiple reasons, and understanding those reasons will help one to understand why different brands are a tradeoff.
Lee filters are handmade, requiring up to an hour of highly skilled labor to create. This hands on approach allows the process to be more closely monitored and adjusted as the filter is created. If you are interested in the exact process that is taken on to create a Lee filter, I would watch this short 15min video for a great overview of the process: Mike Browne visits Lee Filters(YouTube).
Lee filters are known have have very little color cast. When using Neutral Density filters, color cast is typically your enemy, especially with long exposures. While Lee filters do not always eliminate it(especially when considering 10 stop varieties), they are widely regarded to have among the least amount of color cast in their filters among competing products.
Lee filters are made of resin. Why is resin better? Because it allows for sharper images, especially if it is a thin resin and you are comparing this to glass. Resin can be optically more pure than glass. Yes they can be easily damaged, but that is the trade off that you get for the sharper image that some desire. Note - Lee does offer a "Pro Glass Series" if you prefer that route.
I also believe that part of the reason that Lee filters have been so successful and highly regarded is that the mounting system, Lee Foundations Kit is also of very high quality and functions very well. Having both the mounting hardware and filters from one manufacturer does assist in the consumer's perception of quality in a brand name, and in this case I believe the perception is reality.
Lee filters essentially give you the highest quality glass/resin available, thinner and more optically pure than competing products. It has one of the best mounting systems available to match it as well.
Conclusion