Currenet system is *awful* for balance reasons.
Many techs that enable new resource type akso obsolete previous unit that does not need new resouce.
Thus getting muskets or iconclads, or even advanced flight, is 50:50% gamble will you get better units or lose access to current units.
If you are unlucky your military will become wore then before tech is researched.
If anything this hurts AI a lot more then player.
Yeah, I think the system wouldn't be nearly as bad if you always had time to plan. So if there was one tech that revealed coal, and another that gave ironclads, you could at least see that you had no coal before going the ironclad route. Maybe if coal and niter were revealed together - then you can decide after that if you want to pursue the ironclad route, or the musket route depending on what happened to be near you. As it stands now, if you choose wrong, you can be kind of screwed.