The problem with a lot of the TG and MC units is that by the time you get them, you have the 3* versions of the barracks units, so the barracks units still perform better. This can change in some cases when you get your TG and MC units to 2*, but it's really when you get them to 3*. that they come alive.
I'm in the happy position where I have all units at 3* and have Martial Monastery, all training speed and all damage wonders at level 30. I've taken a screenshot of my units screen and added it to this post a little further below. My main go-to units are outlined in green, situationally useful units (niche units) are outlined in yellow, units I only use to spare the others, because I get them for free from my wonders are outlined in red. Units that aren't outlined I do not use.
When you get the 3* versions of the MC units they're pretty much the best units for the majority of encounters (except Drone Riders). Some examples:
Rangers
Rangers are awesome for their extra movement and high initiative. Their enemy defense debuff gives them very strong focus-fire power, even against units they're not strictly designed to counter. You might get a fight where the enemy has one or two heavy ranged or light melee units, but the rest of the enemy team is perfect for Rangers, no problem, being able to attack first you can usually focus-fire these rogue units down with the 30% debuff before they get to attack you (map layout contingent). Only one enemy unit in the whole game is faster (higher initiative) than the Ranger, the Mist Walker.
Frogs
Frogs are great at taking out enemy units from afar, often being able to kill the enemy team without being hit back, but you need to be mindful of the cliff-edge (applies to all ranged units really, but especially the ones with lower base health and attack like frogs). Frogs are great at resisting light melee attacks, so if you're going to let anything hit them, try to make sure it's a light melee unit. Light ranged hurt a bit more and other heavy ranged can hurt more still. The good thing with frogs is that they're faster than the Cerberus (the only light melee unit that can reach them on the first turn, again, map layout contingent), faster than all heavy melee, faster than all other heavy ranged.
So pick your targets well and you can kill most enemies before they hit you. Things like enemy cannoneers should be prioritised over light melee and light ranged, especially if there's more than one of them (they have a nasty debuff, that makes your unit take more damage for the rest of the turn), all other heavy ranged can't reach you on turn one, so if there's only one or two other heavy ranged kill any light ranged next. It's really all about knowing what it takes to drop an enemy unit and prioritising accordingly to kill them before they reach you.
The 3* frogs have a very powerful debuff, like the Rangers, this let's them focus down units that they're not really supposed to counter, like heavy melee or the odd mage. Fights with one enemy mage should be no problem with frogs, sometimes even two, fights with many heavy melee are no problem too, depending on exactly what they are. In terms of frogs, there's weak heavy melee and tough heavy melee. Knights and Swamp Monsters are the tough ones, resist heavy ranged attacks really well. If there's two of these in the enemy team, should still be an easy takedown before they reach you, three of them can be taken down with the right map, otherwise there might still be one weak one left alive that reaches you and does some damage. 1* and 2* frogs don't have this debuff, so mage units and the tough heavy melee are lot more challenging. I'd avoid encounters with the tough heavy melee and mages until you have 3* frogs.
As I touched on above, be aware of the cliff-edge with your frogs (and other ranged units), the point where the enemy team can live long enough to reach your units and do damage. You might be able to beat a certain encounter using auto-fight in a tournament on round 4 with no or just minimal losses, but maybe on round 5 or round 6 the enemy tough heavy melee can get to you before you kill them off and you'll lose half your team. Be ready to switch things up if you can see the cliff-edge coming.
Orc Strategist
If an enemy team consists of mostly light ranged, your frogs will take a bit of a beating before they can attack (the LR always get to attack the frogs first, depending on the map layout), Orc Strategists resist light ranged attacks really well and have more HP so you'll take less losses than using frogs. They also hit light ranged harder than frogs and will usually be able to attack the enemy light ranged on turn one as they'll already have moved in range to attack you. This (facing a light ranged loaded enemy team) is one of the niches that Orc Strategists can fill really well.
Note: I don't have a human city so my viewpoint is for elves only.
