Trouble then is there is no film support to allow the pressure plate to flatten the film.
While I can't comment on the quality of said adapters, I can provide an example of this.
Pentacon Six, 50mm Flektogon, a roll of crappy 200 135 film run backwards (hence the Redscale). Nothing holding the cartridge in but a bit of cardboard, wound onto a 120 takeup reel and unloaded/rewound in the dark.
No supports at all. Have a look on the edges, perfectly sharp columns, but what's happened is that in the middle the film has bowed inwards and it's totally unsharp (I hope that's visible on the shrunk one I'm uploading).
This neg does have sprocket holes but it's a really old scan in a crappy 35mm holder, haven't wetscanned it nor printed yet.
So if you want sprocket holes, yes you can file out the holder, but you'll have to live with stuff like this happening (this is the worst on the roll, some weren't bad, maybe narrower f/ helped the others).
Keep the holder intact and it should be sharp the whole way at any aperture though.