When you're looking for work and need to get a job (e.g. to support a family) spending 10-20 hours working on a single job opportunity is risky because they may just say "sorry, we like you but _____". where _____= {there's a corporate headcount freeze, we don't need that skill, we already have something that does that better, we're not good at hiring}. I still think it's likely a good strategy, just worth acknowledging the inherent risk.

If you have oodles of free time, then sure, it's low risk.