heya Ilmanowar...
good to see you too

Well..
your mentioned board is an option...
possibly it is a little safer...
But when dealing with batteries inside the device (n900)
anything can happen...

concerning
"About what is speaking endsormeans, I think it is not the best/recommended solution because is bulky, inefficient and a bit dangerous for the battery."
Wellllll
I am not sure if you quite get it...
or perhaps you may be referring to something else...
But ...there is no battery inside the n900 with utilization of the LM317...(not in the traditional sense of the word "battery" anyway..)

the LM317 essentially removes that issue.
there is no battery inside the n900...just the shell of the battery..to provide proper normal Nokia BL5J battery connection to the contacts inside the n900...and inside the battery shell ...only leads that go to the LM317...
which from there one can fine tune the output to the n900 battery shell...and at the same time offers surge and power fluc protection...
the power input does not need to be a battery...it can be direct usb power from a wall outlet or computer or battery bank ...

No matter what ...a battery melting down inside the n900 won't happen.
since there isn't a battery in the n900..

what may happen in the form of some unforeseen risk to the external source of power ..is possible and cannot be ruled out.
But considering the protection and distance from the n900..the likelihood is that the n900 will not be physically affected unless the source of the power is in close proximity...
Even still...the step down does have protections to impede such possibility from occurring.

What may happen is the setting on the LM317 could be altered accidentally ..if it is not fixed and set so it must be "unlocked" to move the dial and alter the input to the n900...
well then...it is possible to damage the n900 ..
But this risk to the n900 is purely in the hands of the individual...as to whether or not they wish to put a "safety" on the dial...so it cannot be accidentally moved and hence increase or decrease power accidentally if the dial is jarred, hit or moved.

I dislike messing with batteries...
I do like taking the risk of tampering with batteries out of the equation completely...by simply removing the battery guts...and utilizing the shell.

And I certainly like not having to rely on either fake bl5j's ...old bl5j's....refurbished bl5j's...
but have free choice to pick a method of power source.

the LM317 may not be the most compact and pocket portable ideal...
but it sure helps solve some issues.