When comparing Double Commander vs fman, the Slant community recommends fman for most people. In the question“What are the best file managers for Windows?” fman is ranked 6th while Double Commander is ranked 10th. The most important reason people chose fman is:
Windows, Mac, and Linux are supported.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Multi-platform (Linux, Windows, MacOS X)
You can use same tool in all desktop OS environments.
Pro Source code is available (true free software)
If the developer loses interest there is at least the possibility that someone else will pick up the torch.
Pro Configurable
Lots of options allow you to configure DC the way it suits you best: Tools, Fonts, Colors, Hotkeys, Mouse, Fileviews, Plugins, Layout, Toolbars, Tabs, Icons etc.
Pro Directory Hotlist
Save shortcuts to folders. Organize them in a treeview with submenus. Define a name and sorting for each entry.
Pro Powerful tcmd-like search tool (alt+F7)
Pro tcmd-like multi-rename tool (CTRL+M)
Pro DC uses TCmd plugin API
so you can use documentation from Total Commander for writing plugins. WCX (packer), WDX (content), WFX (file system), WLX (lister).
Pro Closely follows TotalCommander UX
For instance, the 'Settings | Layout' pane is quasi-identical to TCs.
Pro Very sophisticated
Although it is lightweight and simple to use, it can do very sophisticated tasks, like copying files from directories which have a certain extension or file size or have a certain text pattern in them. Also, it's very customizable and stable.
Pro It's that good, you can replace tcmd on windows too
Pro Source code written in (Object) PASCAL
It's all a matter of perspective. I'm not for an argument about IDE's, frameworks etc, but to me that's a big plus. I think it should be a tie, it's either a pro or a con or should that be neither a pro nor a con. It just depends on context.
Pro Works on all operating systems
Windows, Mac, and Linux are supported.
Pro Simple to use
Pro Makes finding commands *by name* easy
Pro Slick
Cons

Con Apparently only one developer
Con Freeze after mouse double-click on directory list item
Con Much slower on Linux and Mac than on Windows
Con Source code written in Pascal language
But this does not affect users negatively. It's just programmers problem.
Con Not developed anymore for Mac
Con Can't edit files on remote FTP servers
Con No (text) file viewer
Con No explicit bookmark support for directories
It though remembers the visited directories and allows to search in this list in most-recently used order and by name.
Con Mainly for key-board-orientated users
The interface is most naturally navigated by arrows and keystrokes. The target market is software developers.
Con Has no menu bar
Hence it is not well suited for visually orientated users which find or remember commands by using a mouse and a menu. Even the fman's hero Sublime Text uses a menu bar.
Con Requires email address to download
Doesn't say what it will do with this data. It is in contrast to the new laws in Europe where only necessary information is allowed to be collected. A download should not require an email address.
Con Still quite buggy
So, for example, sorting only is remembered if triggered by command and not be clicking the table column header using the mouse.
Con Settings can't be found by the GUI
You need to know which files to edit.
Con Default dark theme
No choice between dull-dark or fresh-light.
Con No portable bundle available
On Windows only a net-installer is available.
