As we've found the unfortunate fact about doing Tapa of this size online is that guessing becomes a preferred strategy in many cases. I've kind of been worried that with the ability to make other solves viewable by anyone for this particular contest that it will give the illusion of "guessing is how good solvers get good times". That might be true for 10 by 10 Tapa, but absolutely not in general. Even for the 12 by 12 sizes I avoid just guessing my way through it, at least too early, because there's too large a search space, and my better performances on those have usually been with logic. My total inability to figure out what was going on for March 25th was a notable exception to that, and I did have to rely on luck/"tapa sense" there. I think there's a sweet spot for the difficulty of the larger puzzles that makes them hard but still preferable to do with logic from a speed-solving perspective. March 23rd was dead on in my opinion, and March 25th overshot it a bit.

That said, I don't think it's necessarily the Undo button itself that harms things. I typically forget about it and waste time erasing things manually when a guess screws up, which is still faster than erasing on paper, and online it doesn't leave distracting stray marks too.

Also, I have been going back after the days I bifurcate and solving these logically. Almost every puzzle is fantastically designed, even if it ends up that guessing is the fastest way to get through them, so it's more just a issue with this being a contest than a matter of puzzle quality. However there have been a few days where I've had trouble getting a logical solution without minor T&E. The ones that come to mind are February 25th, March 22nd, and March 25th.