Sitting on the fence - Why I sometimes choose not to use Plone in favour of Drupal or Wordpress
Plone is complex
From a development point of view, Plone is a complex piece of software, at least compared to something like Wordpress. I'm not going to go into specifics here, but for someone like myself who is primarily a front-end developer, sometimes I can lose days trying to add a simple custom feature, that I know I could create in hours on a php based site, probably just with a few lines of code in the template (a practice that is often frowned upon, but usually harmless and more importantly gets the job done). However much of this is down to my skillset and understanding (and probably my natural abilities to a certain extent) which brings me on to the next point.
Development resources - a chicken and egg situation
I have to hold my hand up and admit to being a bit of a "web monkey" when it comes to development - i'm self-taught, beginning with hacking around classic asp and php scripts and moving onto object orientated programming only in the last few years. With Zope and Plone the entry bar is higher - there is a whole framework to learn if you want to develop custom features, which is a good thing from a maintainability and system architecture point of view, but there is a significant time investment involved for someone wanting to transfer from something like php. Even an experienced Python developer will have some learning to do to get to know Zope and Plone.
This means that I struggle to find local freelance resources to work on a large Plone project with me. I also struggle to find sysadmins with zope/plone experience, and not being a sysadmin myself have found myself in a few hairy situations. I know of at least two ventures that have got into difficulties because of resource issues - I think partly because Plone can be a victim of it's own success - there are lots of Plone developers, but they are all busy!
Hosting Plone requires specialist hosting - or rather you can't run it on the majority of locked-down budget shared hosting. Many of my clients already have hosting arranged and don't want to move, so something like Wordpress suits their needs better, for a simple site.
Overkill for simple sites Sometimes a site only needs to be dynamic for the sake of a blog/ news section - I find plone is overkill for this, and often prefer to go with something more lightweight, even (gasp!) a bespoke CMS when I need to bolt extra functionality onto an existing site, and there is no budget for a rewrite.
So when would I use it? Without a doubt Plone makes a fantastic intranet, out of the box. I eat my own dogfood here - I use an unmodified Plone 3 site for my own company intranet and document management system. Drupal/ Wordpress do not even appear on the radar in comparison for this task. I would also use it for any site/ application that has a need for complex workflow, membership and groups.
Lastly, i'm happy to use it for any project where I can team up with an experienced zope/plone techie to help with the more low-level stuff. I was recently blown away by seeing how an experienced Plonista at Team Rubber quickly dealt with some temporary extremely high read/write traffic on a Plone site by firing up a whole set of zeo clients on amazon EC2 - a scenario that i'm sure would have been a nightmare to deal with on a drupal or wordpress site, and i'd be equally out of my depth without help.
In summary I hope this is useful and doesn't upset the Plone evangelists too much. Plone is a serious, well architected, secure system that leaves the competition standing in most cases. I think by "competition" I am talking about proprietry CMS and intranet systems that cost ten or hundreds of thousands of pounds, not the likes of Drupal or Wordpress. If you don't want to do much customisation it makes a good choice for smaller projects, but if you do you are going to need a developer (or invest in growing one of your own), who is a. available, b. you can afford, otherwise you might be better to go with something else, with a lower development entry level.