I recently have run into the problem where I can't tell the difficulty of Core Miner. I play the game repeatedly and know how everything works under the hood, so I'm extremely familiar with it - that is really bad for balancing. I have been getting friends to play-test my game and the first thing they mention is that it is too difficult or that getting upgrades takes too long. So I figured each time I tweaked features because they were not quite right - I was actually making a mistake.
Wind back the clock to when I was working in a big company: when I was working on the Transformers Universe project, I noticed that although everyone was working on the game every day there were still some folks who had not played through the game. So this meant there was always someone "new" who you could watch play the game and get valuable feedback from. Oh and you could always grab someone in the office who was working on another project.
Now I'm a solo developer that pool of people is just myself. I try to play the game as much as I can to find problems, see how fun it is, etcetera. Usually that's fine. I always spot something I want to change and add it to a "To-Do" list - including notes on balance. I think in future I need to ignore balance issues and leave finding those problems to a fresh pair of eyes. I'll likely need to keep tweaking the game and watch people play for a month or so to get it to the point where it feels right.