It's not really that Ostheer T4 is bad, it's just that tiger tanks are so much more fuel efficient. A tiger is better and costs LESS to get your first one, making it the pretty obvious choice in most situations.
Mines are most important for USF. If you spam m20 mines all over the place they could easily win the game for you.
Soviet mines are great just because a lot of soviet strategies don't require you to spend munitions on much of anything else so you can spam them like crazy to bleed your opponent.
Ostheer mines are nice but like you said probably not as important as getting upgrades on your units. It's much more circumstantial.
OKW mines usually don't pay off all that much. Your munitions are probably better spent on shrecks or the occasional grenade.