Hi Guillaume.

Jira uses proprietary Atlassian wiki markup for its rich-text and Octane (and most other products) use HTML for rich-text.

Conversion from wiki markup to HTML is simple and straight-forward, it is provided by the Atlassian Jira API.

However, conversion from HTML to Jira wiki markup is NOT straight-forward!

There is a very basic conversion capability available, which is what MF Connect uses by default.
Basically, only font size, bold, italic and similar are converted, most other formatting is lost when sync'ing to Jira.

That said, Atlassian informally publishes the Atlassian Renderer API which can be used to achieve better conversion from HTML to wiki markup. MF Connect will use this alternate, more powerful method if it is enabled in the Jira data source.

However #1:
The Atlassian Renderer is not perfect and has conversion gaps and some bugs!
MF Connect works around some of these, but cannot do so for everything (the Atlassian Renderer is closed source and not officially supported by Atlassian).
As such, in average, results via the Atlassian Renderer are better than without - BUT some HTML/rich-text constructs can completely "confuse" the Renderer and lead to unexpected, incorrect results. There is no way to clearly define the failure triggers (as it is closed source).

However #2:
Atlassian wiki markup is notably less powerful than HTML!
Some every-day constructs with HTML-based rich-text simply cannot be converted to Jira because the wiki markup lacks the features for it.
A prominent example is that with wiki markup, one CANNOT freely define background color of an arbitrary text selection.
The ability to create tables is also much more limited with wiki markup than compared to HTML.
There is no workaround for this limitation of the Atlassian wiki markup.
-> https://jira.atlassian.com/secure/WikiRendererHelpAction.jspa?section=all

Conclusion:
With the wiki renderer turned on in the data source ("Use Wiki to Store Rich Text"), most Jira items will receive better rich text results - but at the price of some fewer Jira items receiving worse/unpredictable results.