It seems to me a lot of the negative thoughts towards the Enfield rifles in the US goes way back to the 1960s, when the less-accurate part-worn rifles came over in the great flood of surplus between 1953 and 1968. Many of the guns had bedding issues. At the same time no one knew much of anything on how to regulate the rifles, it was not until Reynolds book came out in 1960, which had a small printing run that the information was available. Frankly I have never seen a copy of Reynolds pre 2nd edition of 1968 in the US of A. So most folks did not know how to rectify the issues.
Add to that the rifles coming over Pre 1968 were almost all throat worn, so the issue with nitro accuracy out of a worn throat could come into play post 1968, when most of the large imports of 303 kind of dried up. Yes there were periodic imports of the ammunition, but post 1979 to around 1984 virtually none came in as it was bought up for various 3rd world areas.
SMLE rifles have a tendency to string vertically, which seems to be its worst in the 200 yard range. Mixed lots of ammunition make it worse. Lots of the surplus that did come over prior to the final large sale was mixed lots, so the older ammunition lots, particularly the ex-belted ammo had a reputation for erratic vertical dispersion.
When the war reserves stocks started to come in to the US, like the Australian SMLE rifles and the various like new or near new No 4 MK II, it was quite a shock to many folks how accurate the rifles could be. But for the non Enfield specialist, the reputation was already formed.