While developing with yahoo pipes,  there are a couple of undocumented issues that Developers should be curious about,
so that you should decide if yahoo pipes is right for you or not. 
First one is how often yahoo bot updates  your feeds,
According to my experiments (I logged  rss requests made by yahoo), Yahoo bot updates  rss each 12-25 min.
It seems like frequency of update is directly proportional with yahoo pipe request made by users. the more users call pipes, the more often yahoo pipe updates rss sources. 

Seccond one is Time issue ,
Every developer who has played with pipes for  a while will recognize there are small but important details with yahoo pipes time variables. I will try to explain that issue with an example,
For instance, you requested a news rss and it returned you rss with item element pubDate like
<item><pubDate>16.02.2009 08:33:00</pubDate> …
At first sight, there is no problem with that time format yahoo will receive this and deliver you back.
But since there is no  Greenwich mean time (GMT) description in that time format, yahoo automatically assume this date is from its own region and converts date to pacific time format so return this date as US time.
If you have one source and you don’t care about the time it is ok.
But if you are using multiple sources and especially if the control of generating rss is not in your hands, you should think about this issue very carefully before your decision. Let me give another example.
Assume you received another item from a rss source like this,
 <item><pubDate>Mon, 16 Feb 2009 11:26:00 +0200</pubDate>.. 
(My hometown is in +0200 GMT) 
And we already have previous item, 
<item><pubDate>16.02.2009 08:33:00</pubDate> … 
When you tryed to merge  rss items and order by date, you will see pipes will assume 08:33 as pasific time (cause there is not GMT), and order it earlier from +0200 time. 

 Third issue is updating pubDate value on RSS item
Assume you need to change pubDate element of received rss, the way for doing this is not the same way you changed rss title or description. This is more complex, you need to regenerate the item for changing pubDate.

For an example of updating pubDate and other rss elements see yahoo pipes rss element update example. 

I appreciate yahoo pipes, it is easy to develop and maintain but have few issues with reliability and performance, in order to make sources suitable for yahoo pipes in my project, I made a proxy between yahoo pipes and news sources. (Receive news, make changes for yahoo pipes, feed pipes.)
I think pipes is a cool idea (for those who use Quartz Composer :) will understand me better. ) 

in the following days i am sure yahoo will improve pipes and make it much better.  :)
If you want to share your tips, don’t hesitate to review or contact,
Bests

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