A friend told me its very hard to implement a bughouse chess engine because it requires much more processor power than a regular chess engine.
Are there any bughouse engines out there ? if not, what are the reasons ?
There are certainly several chess engines, but as a result of the additional complexities referenced by @TonyEnnis, these engines are weaker than reasonably strong players. It's extremely difficult to take into account the different scenarios that can arise from two positions as pieces switch from board to board every move. The resulting search tree increases at an exponential rate even greater than that of standard chess.
The engine that I'm most familiar with is Sjeng which claims that it has an ELO of around 2000 on FICS, equivalent to a strong human player.
Sjeng is your best bet but play with an engine is very weird. Typically a human + engine team will have their play revolve around getting up on time (sometimes even by only a few seconds). Then the human player will tell the engine to move fast, and then sit. Note that since the computer can play decent moves instantly, that effectively means it is impossible to sac material against the human player unless also up on time -- even if the sacrifice is very strong and leads to an imminently winning attack or quickly recovering the material. The human player will accept the sacrifice(s) and sit, then let the engine win on time or use the extra material on the board to win. For these reasons none of the strong bughouse players will accept games with engines anymore, really.
I don't know of any bughouse engines.
In addition to playing the standard game...
Sunsetter is best seems, but Sjeng (especially 11) is good for many purposes too.