I found it files OK if your file is brand new (sharp). The difficulty isn't so much the hardness, more the 'slipperiness' — the file wants to glide off without biting (and nick the adjacent steel), so you have to press on the file enough to make it cut. A carbide burr cuts it just fine, hardly noticeable difference from brass. And of course abrasives (cartridge roll, dynafile) work too.

Luckily I was warned about the difficulty in filing. I made and filed a couple test joints before I committed to making a whole frame.

Funny story, after I built and painted the frame, hung parts and rode it, I decided I'd made the toptube too long. So I cut the entire head tube off, re-mitered TT & DT some amount shorter ( I think it was a half-inch) and put on a new HT. Luckily paint was free to me, they didn't charge me for the amount of Imron I used. So anyway, I still have the original HT with the TT & DT joints. I sectioned it to see how I did. Full penetration (tiny inside fillet all around), no voids, and if there's any undercutting of the steel, I can't see it. Overall I am pleased that it came out so nice — I've seen lugless frames from much more experienced builders (e.g. Alex Singer) that show a substantial trough around the fillet from filing. But I had smoothed a mile of fillet on Santanas before I made my own. That's the job they made the apprentice do, not because it requires less skill, only because it's an unpleasant job that they (the two journeymen) didn't want to do! Those very first Santanas when we all were first learning probably aren't very good.