Hello Francisco,
I am using Feng since about 2010 (CE).
Up to now there hasn't been even a good enough alternative to feng for group task management by far.
That is why the mobile work on it wasn't the deal breaker.
However, Since producteev has been aquired by jive and gone "free" while it has also a free fully working mobile app, the barriers to change have gone done considerably.
My best recommendation for you, if you want feng to continue growing and not be suddenly dropped down because of major changes in the market, is to have a free mobile version for iPhone an android.
If it cost too much, invest in an API and let others do it.
I'll confess, that at the moment, if there was an "import" to producteev from feng, I would have used it in a heartbeat, although they are not yet fully specific feature compatible with what I need - the mobile has become crucial enough that other stuff can wait.
Hope it lands where it should