I think that the key to using weed membrane (mypex for short) is to …
1. use it as a temporary cover and never cover it with soil, compost or wood chips for the reasons Charles gives:
2. avoid simply cutting with scissors which gives a frayed edge. I sealed the scissored edges of mine by running a lighted match along the edges but ‘cutting’ with a blow torch is easier.
The only weed that my mypex did not kill was bindweed, but in another area the polythene sheet was also not effective on the bindweed. Fortunately I did not have horsetail so I cannot report on this.
One plan might be to …
1. cover the whole area with mypex until you are ready to build your raised beds and fill. This will start the weed eradication process:
2. remove the mypex, put down the frames and fill with compost; this is best done as soon as possible to allow the worms to get working:
3. relay the mypex over the whole area, paths and all, until you are ready to plant up in spring, preferably late spring or early summer using module raised plants.
4. after removing the mypex put chippings or whatever you prefer on the paths. I used wood chips as they are easy to hoe.
I used a similar approach on my allotment, although I did not use raised beds. Photos here. In late spring I removed the mypex and planted module plants. I had to do a bindweed hunt every week at first but otherwise maintenance of the beds, through regular hoeing, was very easy.
The mypex is reusable but the method is so successful that I will need to get another weedy plot to reuse it!