It's not difficult to see why people are tempted by such cheap parts when you consider the cost of genuine:
Current retail price (incl.VAT) for upper arms is £67.33 each. A right-hand lower wishbone will cost you£176.44, and a left-hand lower is (weirdly) £181.24.
If you're attempting a DIY job, then you may think it's worth taking a risk, as your time doesn't cost you, and it's not your reputation and finances that suffer if you have to do the job again for free "under warranty".
I have been working on Alfas since before the 156 was introduced, and over the years have changed dozens (probably hundreds, actually) of wishbones. When pattern replacements started to appear, we tried many different makes, with varying levels of success. Experience has shown that most cheap copies are noticeably inferior, and we now only use Original or TRW. The reason for using TRW is that they are currently the OE manufacturer- the parts are identical, except that the TRW ones have the Alfa logo and part number ground off. Having said that, I have even rejected some TRW lower arms, as they were clearly not the same as the originals (an easy way to tell the difference between the "good" and "bad" ones is to look at the rear mounting bracket; it should be a smooth, shiny section of aluminium extrusion, not a dull, rough casting).
Problems that I have encountered with cheap lower wishbones include badly formed threads, brackets out of alignment, weak threads (particularly in the cast aluminium rear bracket), balljoint boots incorrectly fitted, and "captive" nuts making a bid for freedom. And that's just when fitting the things.
In service, issues include premature wear of balljoints and rear bushes, separation of bonded front bush, and perishing/splitting of balljoint boot. (by "premature" I mean within a year of fitting).
As for polyurethane bushes (Powerflex etc.), they work well in certain applications, such as rear arms on 916 GTVs, and rear hub bushes on 156s. However, they are not well suited to 156 style front wishbones; the main problem with them is that they have little or no capacity for lubricant retention, so very quickly dry out and start squeaking. I refuse to fit them now, but if you like stripping and lubricating your front suspension every three months, then they are ideal! The other issue of course is that if the balljoint fails, you're back to square one, as it's part of the wishbone.
On the subject of lubrication, if you really must buy cheap upper arms, then it's a good idea to lubricate the bushes prior to fitment. Unlike the bonded rubber bushes on the lower arms, the uppers use a pivoting bush, which uses rubber as a bearing. Cheap arms often have very little, and often no, grease applied. It's quite easy: pop the metal "top hat" sleeves out, apply liberal amounts of a suitable lubricant (such as Castrol Red Rubber Grease) and press the sleeves back in using a bench vice.
The underlying problem is that the 156-style front suspension is a poor design, and coupled with inconsistent quality control, is the reason why even original parts sometimes don't last as long as one would expect.