These products vary in price and offer a wide range of features. In addition, free public-domain firewall software is also available. Users have to directly interact with the firewall. Client software has to be replaced with proxy-aware clients. Proxies aren't available for a number of services.
Unproxiable services are difficult to process.
Performance is limited by the FWTK's design. There's no automated system administration assistance, thus requiring significant Unix knowledge by the administrator. The vendor no longer supports the product. These limitations and bugs have been addressed by a number of today's commercial, albeit more expensive, firewalls. For those of you who are into research, an overwhelming number of articles that evaluate firewall products based on various criteria are publicly available.