From a much broader perspective: You only have three scoring units. In the primary detachment, you're rolling a Deathstar in a Spartan
and then only 10 Vet's in a Rhino. How's that gonna play out? That Rhino will get popped first turn before it even gets to move or maybe after your first move. You get the idea. It's pretty easy to erase 10 Space Marines off the table. After that, you have a lot of minimum sized squads foot slogging across the table. The only anti-armor you have in your army is the Spartan, Xiphon, and then in the detachments, a jet pack robot that can jack stuff up in close combat, or from the Militia the Medusa and/or 3x Laser Destroyers. For a 2,500 point list, that's pretty light. Could you destroy a Spartan with shooting? How about a Deathstar? The Xiphon is awesome, but fragile, and will at best arrive on turn two, and at worst turn 5. I'd say unless your heart is set on it, don't run an allied detachment, and get some more troops in your list, like 2-3 more Tactical Squads with Rhinos, and take the Rapier's as Legion Rapiers. Then try and squeeze in one more tank, like a Predator or if you have the points, a Sicarian (Battle Tank or Venerator- they both work). If you changed nothing and wanted to run these lists as is, then I'd say take the Militia over the Mechanicum. The Mechanicum detachment doesn't do much for your T-Sons, and your T-Sons don't do anything for the Mechanicum. The Militia list at least gives you more scoring Troops and much needed anti-tank.
The Mechanicum Detachment has a T3 unit with 20 models with a 4+ foot slogging. That's... not good at all. They have victim stats, and that
LD 7...
FnP 6+ doesn't help you when you have T3.
The Militia Detachment is a little better, but the Rotor Cannons are not very good. Is there anything else they can take? I don't have that
HH Book, so I know nothing about the Militia. I do know a S3/AP6 Salvo 3/4 gun sucks... hard.
Detachments are one
HQ, one Troop, and one additional Elites, Troops, Fast attack, and Heavy Support. You have one too many Heavy Supports for your Militia, and you can't run them as your Primary Detachment because you are running a Primarch (Primarchs must always be the Warlord, and the Warlord must always be in your Primary Detachment).
One last thing: The Limitations on the RoW Primarch's Chosen: The number of units with the Legiones Astartes special rule in this detachment must be greater then the units without the Legiones Astartes special rule in total. The only units with the Legiones Astartes special rule in your Primary Detachment are the Sekhmet Terminators and the Veteran Squad. It's important to note that the Spartan is a Heavy Support choice, and not a dedicated transport. Magnus, The Spartan, and the Xiphon do not have the Legiones Astartes special rule. Crazy that a Primarch doesn't have the Legiones Astartes special rule, but /shrug. All the Primarchs are like that.
Every time I make a list, I ask myself the questions "Can I shoot a Spartan to death in one round? How will I deal with a Primach? How will I deal with Flyers?" And lately, "How will I deal with Artillery?" If you have a solid plan for all four of those questions-- a realistic plan too-- then you're good to go! Sometimes though, it's Rule of Cool and you just have to play your army, your way, and that's OK.
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