Having almost 30 years experience of ferts:-
a) vulnerable ferts are a big mistake even at imps while at mps -200 is a frequent disaster. Corollary : for more than a decade I have resisted playing any system which required a vulnerable fert. Pass when vulnerable is a great option!

when you own the partscore, opening a fert is likely to be antipercentage and unsurprisingly makes finding the right partscore that much more difficult (pre-emption works against you here fairly obviously). These are the hands you are most likely to lose out on particularly a secondary (eg 44) fit or fit based on primary length in fert's hand;
c) when you have a real dog and can up the ante with a 1S or 2C fert at favourable, your opponents labour to bid slams (much less grands) : the pre-emption works in your favour;
d) against good opponents the fert can give them information in the play of the hand;
e) like any other pre-empt a fert may stop them bidding some games and push them into others (which may make) thereby causing a randomisation or swinging effect;
f) the only "2-way forcing pass" I like is at radical extremes of the range and is pretty much prohibited by ACBL: ie Pass is either 0-4 or 16+ so that there is an injection of additional risk in the defenders' destructive actions (their "weak jump overcall" on a tactical basis on 15HCP now looks stupid when it is their hand, or moving on some 44 generates too high an auction etc). Note if HUMs are permitted the 2-way structure is permitted but in most jurisdictions it is increasingly rare;
g) other 2-way forcing or non-forcing pass systems are "hole-fillers" for your constructive system but place your side at a radical disadvantage when those hands occur in both theory and practice;
h) Richard & Ron are correct as to statistical frequency of occurrence of the 8-12 HCP range on any given deal, but of course the corollary as to whether frequency of occurrence ALONE is sufficient justification for allocation of bids is a separate matter. Funnily enough it makes sense at pairs where frequency is king (but now you as superior players will be disadvantaged by rarely playing in the same contract as others) but size of result matters more at imps (clearly) and my experience is that it is less clear here (in fact doubtful). This does not mean that methods in 3rd & 4th seat should be exactly the same as 1st and second - au contraire as who would wish to fert in 4th seat to give the most obvious example!
i) The experience is based on records of every auction that Bob & I have played in ANY competition and bidding competitions and practice hands over 20+ years as he maintains such scrupulously!!! It is still anecdotal but I suggest there are few if any southern hemisphere players with an equivalent database.
Against better players not trying to get clever the fert at teams is an overall loser, but a frequent winner against their slams (sort of like a mini -2NT slam killer...).
j) ferts appeal to juniors to get a chance to bid on really bad hands and jam the opponents but like big club/strong pass, it is the remainder of the hands (midrange) which derive benefit.;
k) If you are going to fert, to do so "safely" via 1C/D is losing bridge as it gives the opponents a fielder's choice to bid to their par contract with improved information or to attempt penalties with no pre-emptive risk: it is VERY easy to design systems which take advantage of 1m fert to clarify your own constructive bidding;
l) subject to the usual adjustments my view is to go fairly mainstream to allow maximum exposure with bids: 10-15HCP approximately which requires less fiddling with your judgement in responding -developed over many years as well, while allowing play in most events;
m) I find it bizarre that so many organisations have placed a regulatory ban on 2-way methods and/or psyching the strongest possible bid both in terms of taking the tactical nous out of the game and placing too great an advantage in the hands of the defensive side.
Finally, I find it remarkable that organisations have gone out of their way to ban intelligent system design: eg encryption, limiting the "number of systems" a partnership may play when it is clear that at unfavourable say a system based on ROMEX big bid rinciples has advantages, all vul a big club is reasonable, n/v a strong pass is playable and at favourable a strong pass and fert actually sensible!! Particularly in the latter instances fairly obviously the 3rd and 4th seat bids are different from those in 1st and 2nd which also fall foul of the regulatory requirements!
Of course, being that rara avis, a system maven and designer, I see the benefits but that does not necessarily make it practical even were it allowed as very few partners are able to cope with 1 system both on memory and derivation/application of principle in the heat of battle, much less many!
IMHO that is not an excuse for regulatory bodies to deny the opportunity!
regards