I am just an ordinary mac user like most, not a programmer or expert. So I can't comment on why applications don't work or who is to blame. I can only tell you about my user experience.

More than a year ago I started using Default Folder X for quite some time and I really started to like it. I especially loved that I could finally specify a standard open/save folder for each application to show up right away in the dialogue. A feature that I would actually have expected Apple to come up with sooner or later (rather than never).

But however much I liked the comfort Default Folder X gave me the truth is that it freezed my open/save-dialogues many many times. Often fore-quitting the application in question was enough but sometimes I had to do a hard reset (holding the start button for some time).
As I said I don't know why it did that. All I know is: eventually I uninstalled Default Folder X and then the problem was gone and has never returned since. Too bad because I really liked the comfort of Default Folder X.
I must say I haven't tried version 4. Maybe it runs better now. Who knows...

Only one more thing: since I am not a programmer I can't say how much work it is to write and maintain Default Folder X. But if I compare it to other very useful applications I bought (like Little Snitch, 1Password or TinkerTool Systems) it seems a bit expensive at 35$ in my opinion.