Most mugen characters, and stages work as is.
You do have to tweak a few things here and there just like any version of mugen.
It displays a specific error when it does error and tells you what line of code it died on.
It has almost all the bells and whistles people have wanted within mugen.
It takes a bit to setup the way you want, due to the vast variety of options
Here is just a tiny bit of what it can do (taken from the select.def):
;---------------------------------------------------------------------
[Characters]
;How to add characters
;---------------------
;Use the format:
; charname, stagefilename1, stagefilename2, ...
;
;eg. For a player with filename "kfm",
; and stage filename "stages/mybg.def"
;type (without the semicolon):
; kfm, stages/mybg.def
; IKEMEN feature: There can be more than 1 stage assigned to
; character. In such case stage will be randomly chosen.
;
;If you want to load a different def file, you can enter it
;as a directory plus the def file. This example loads up
;chars/kfm/alt-kfm.def:
; kfm/alt-kfm.def, stages/mybg.def
;
;If you put "random" as the stagefilename, then a random stage will
;be selected for that player. eg.
;IKEMEN feature: Not really needed since no stage = random anyway.
; kfm, random
;
;Optional parameters may follow on the same line, separated
;by commas. Each parameter has the form:
; paramname = paramvalue
;where paramname is the name of the parameter type, and
;paramvalue is the value to assign that parameter.
;The optional parameters are:
;
; - music
; Set the paramvalue to the name of the music file to use
; as the BGM for that character. This overrides the bgmusic
; parameter in the stage's .def file, so you can re-use the
; same stage for multiple characters, but have a different
; BGM playing for each person.
; IKEMEN feature: There can be more than 1 music parameter
; set for stage. In such case track will be randomly chosen.
;
; - zoom
; IKEMEN feature: Set the paramvalue to 0 to avoid stage zoom
; even if it is enabled globally. Useful for characters that
; don't work well with zoom (for example bonus characters)
;
; - boss
; IKEMEN feature: Set the paramvalue to 1 to include this
; character in "Boss Rush" mode.
;
; - bonus
; IKEMEN feature: Set the paramvalue to 1 to include this
; character in "Bonus Games" mode. Bonus characters also
; won't show up as Simul team if faced in arcade mode.
;
; - ai
; IKEMEN feature: Set the paramvalue between 0 and 8 to force
; AI Level regardless of difficulty and AI ramping settings.
;
; - winscreen
; IKEMEN feature: Set the paramvalue to 0 to avoid displaying
; win screen after beating this character.
;
; - includestage
; Set the paramvalue to 0 to avoid including this stage
; in the stage select list (in VS, training modes, etc)
;
; - exclude
; IKEMEN feature: Set the paramvalue to 1 to avoid including this
; character in 'rows' and 'columns' screenpack selection grid.
; (similar result as hiding unselectable AI only characters in mugen
; without need to create tons of unnecessary select.def slots to do so)
; Excluded characters are not added to randomselect pool.
;
; - hidden
; IKEMEN feature: Set the paramvalue to hide a character in a certain
; way:
; 1: cursor can move to this cell, face and face background
; are not rendered (same result as hiding selectable characters
; in mugen by placing them outside visible grid, but without need
; to create lots of unnecessary select.def slots to do so),
; 2: cell can't be selected, art is not rendered (slot is ignored until
; character is unlocked).
; 3: cell behaves like randomselect slot until character is unlocked.
;
; - order
; Set the paramvalue to the ordering priority to give the
; character in arcade mode (1 by default). A smaller value
; means you will fight the character sooner. You will never
; fight an order 2 character before an order 1 character,
; and never an order 3 character before an order 2 one.
; For example, you might want to set your boss character
; to have order=3. The default order value is 1 if you omit
; this param. See *.maxmatches under [Options] for how to
; limit the number of matches per order priority.
; IKEMEN feature: Max 30 orders limitation is lifted.
;
; - 1, 2, (...)
; IKEMEN feature: paramname points to the match number in
; arcade mode. paramvalue overwrites character that you would
; face following normal 'order'. paramvalue should match exactly
; the charname text typed after "Insert your characters below"
; line, so it can be either full DEF path or just folder name.
; This functionality introduces simple solution for implementing
; story like arcade mode similar to the one in Street Fighter
; Alpha 3 where characters always face their rivals along the way.
; If you're playing in Team mode than paramvalue character becomes
; opposite team leader, other characters follow normal order.
;
; Some examples:
; kfm, stages/mybg.def, includestage=0
; kfm, stages/mybg.def, music=sound/song.mp3
; kfm, stages/mybg.def, music=sound/song.mp3, includestage=0
; kfm, stages/mybg.def, order=3, 3=kfm, 6=SuaveDude
; kfm, stages/mybg.def, stages/mybg.def, music=sound/song.mp3, music=sound/song2.mp3, hidden=2
;
;You can also add a randomize icon to the select screen. To do
;this, put the word "randomselect" on a line of its own, with no
;extra parameters.
;
;Insert your characters below.
by 
