Have found with regards to reg fee domains most end up being worthless. Of those I've registered over the last couple of years I'd say the drop rate is 80-90%. This is not random stuff either, huge amount of research done on each name. At the end of the day sales and parking covered about 60% of reg fees in the first year. That was using Godaddy coupons as well.
Sometimes it is worse to get a couple of sales than no sales at all because the "couple of sales" can result in you renewing domains that might generate sales but does not return a profit. Over time that can result in someone losing a lot of money.
Drops/aftermarket purchases is a much easier game but around 30% of purchases will still result in a loss from my experience. Failing is absolutely part of the game.