What is it’s main drawback ?
It’s almost laughable to ask, not only do we struggle to find enough words to describe it - new words are being invented.
... and it does this, and if you go there - you can do this, this . . . (echo)
MODX vs joomla isn’t a valid expression, like Firefox vs IE - forget about it
But, what are it’s "evils"?
Say, has it ever dawned on anyone to pack the installation with more templates ? More examples of how snippets and chunks are used ? It did take a wee bit of an imagination to get enough of a vision of it’s capabilities to want to read some of the tutorials and skim through the forums.
I first learn’t Mambo way back when by pulling it apart . . . I started that habit when I was ’two’ and pulled a tapedeck to pieces, at ’five’ I’d graduated to remote control devices - manuals are more interesting when your ready to put something back together again, but I don’t think I’m the exception.
Just a thought, I typically thumb as far as I can because I have demanding client’s and not a lot of time ~ like, I didn’t even have enough time to read my birthday cards

, reading tutorials is like taking a vacation

. . . I’m sure others might make a faster determination of modX based on how much they accomplished with the installation files, which isn’t a lot unless your able to scratch around for a few days and learn from these forums and tutorials - which itself, you gotta admit - isn’t fantastically cohesive.
Here’s an example, after I completed the installation - I found items called ’chunk’ and ’snippet’. I searched modXcms for ’chunk’ - and got this
Syntax @CHUNK chunk_name
Binds the variable to a document. Where chunk_name is the name of the chunk. The returned value is a string containing the content of the chunk.
This binding is very similar to the @DOCUMENT binding with the exception that it will bind the TV to a chunk (or htmlsnippet).
Example:
@CHUNK MycontactForm
Yes, chunk made a lot more sense to me after reading that
Perhaps a newbie search tool - search for "chunk" and the results give a quick definition, and here’s an example of a chunk, and here’s an example of how you insert a chunk - and here’s a page that uses the chunk.
Honestly, if I’d had quick access to the above for chunk and snippets my learning time would have been a fraction of the time I spent navigating all the search results and forums to get the bigger picture, so, how many newbies throw their hands up in the air and move onto the next cms option ?
Now, back to it’s "evils" - and honestly, if it doesn’t have any, then you can add ’difficult to locate, navigate and understand the educational literature’ which I’m sure given your collective experience with modX really shouldn’t be that hard to address