There are fundamentally four types of fly reels: 1. Reels with spring and pawl "checks" that prevent over-run but offer little to no "drag", classic Hardy's for example. 2. Draw-bar reels like Abels that have a threaded bar and coil spring that adjust pressure between a plate mounted with a compost cork or other material like carbon and the flat inside of the spool. These are not sealed but are easily user maintainable and strong. 3. Increasing popular stacked drags alternating synthetic polymer or carbon discs with stainless steel in a "sealed" housing. These use both side of the discs and, depending on size, use several discs thus offering strong, efficient, compact drags. How truly sealed they are is questionable but they require far less user maintenance than draw-bars. Hatch, some modern Hardys, notably Fortuna and Orvis Mirage would be examples of this design. And, 4. "Hub" drags. This category, the majority of reels, is diverse but involves a drag assembly concentric with and either at the base of the spindle inside the reels' housing or in a hollow recess within the spindle itself. These can be strong and sealed as in Nautilus NV or smaller surface area, un- or partially sealed as in so many reels. I noted on Ross Reels web site that just about every model has a variation on this theme and each a seemingly different design! Some are flat some are cone shaped but most are relatively simple compression designs not unlike spinning reel drags.
A saltwater or steelhead angler may feel the need for a powerful smooth drag and gravitate to one of the draw-bar models or stacked drag systems. A trout or bass fisher may like a drag to more quickly bring his quarry to net for safe release but rarely will be setting his reels' drag to a potent level so is fine with most any drag design including the finger palmable click check type.
When comparing drags, pay attention to smoothness, start-up energy, linearity of setting and fine-tunability. All reels should be kept lubricated as specified by the maker...which may include NO lubrication, should be stored clean and completely dry and with their drag setting at zero. Quality reels will have minimum surfaces that can entrap an errant coil of fly line, tight tolerances that won't suck tippet into their gaps, spool aspect ratios that promote easy and intuitive uniform retrieval of backing and line and virtually no flex in their spool/housing assembly. The spool release mechanism should be stout and as failure proof as possible, I have grown to favor the treaded captive nut type. A good fly reel need not cost a fortune but won't be inexpensive either and, properly treated and maintained, will last a lifetime.