Is there a word or term for an attempt to simplify but which complicates instead?
You write, "The tool that a coworker has recommended has a very simple interface. The fallacy ... is that the tool will make easy tasks easier at the expense of making harder tasks harder", and are looking for a phrase to describe the situation.
The phrase "steep learning curve" might apply. According to wikipedia, "The familiar expression 'steep learning curve' may refer to ... a pattern in which the marginal rate of required resource investment is initially low, perhaps even decreasing at the very first stages, but eventually increases without bound", and "the metaphor has become more commonly used to focus on the pattern's negative aspect, namely the difficulty of learning once one gets beyond the basics of a subject." However, the page also notes "confusion and disagreements even among learned people" about this phrase, so it is not of unmixed value.
You might adapt the phrase, and refer to a "learning cliff" to emphasize the difficulty of doing more-involved tasks with whatever tool you are talking about.
If you are quite sure of your facts -- that is, if you have used the tool in question, for easy and difficult tasks both, and also have used competing tools for similar tasks -- you'll be better off providing solid comparisons instead of using rhetorical devices to make your case. That said, here are some phrases that may serve your purpose in product quality discussions: "This product has a beautiful camel's nose, followed not far behind by a giant camel's ass" (referring to "camel's nose" paradigm); "This product has a Lamborghini body grafted onto a model T chassis"; "You can do anything you want with this product, as long as it's what x wants you to do with it" (x = giant software company name).
While "deceptive" has many connotations, "deceptive" or "deceptively complicated" might help trigger the right alarm bells in your situation:
de·cep·tive
adjective /diˈseptiv/
1.Giving an appearance or impression different from the true one; misleading - he put the question with deceptive casualness
(deception) misrepresentation: a misleading falsehood