OK restatement of theme/value proposition of the Iron Hands.
-Based on older fluff / theme from Index Astartes with forces are based on the campaign on Medusa itself fighting off the 13th Black Crusade.
-Wider and more general access to specialist Titans than most Marines due to close ties to the Mechanicum. Less flexible air assault assets during the campaign. (e.g. Titans or Flyers)
-Deliberate methodical mechanized infantry that can take a bunch of BM to the face and still not break. Drawback is that your formations are larger therefore require more mutual support than average (e.g. less forgiving)
-Reduced fast attack options for increased ability to drop pod deadly formations (you can drop pod terminators into the opponents deployment zone). Speeders are present as they are the main source of Marine MW attack and without them an armoured force would eat them for lunch. Assault marines were specifically used to launch counter attacks from the fortresses themselves (you can counter charge from the fortress afterall).
-Formations are more geared towards being an "all-comers" ability with a wider mixture of AP/AT. Drawback is that they are not as effective against homogeneous targets until you're in FF range. Deliberate choice of harder to synergize ranges/abilities for shooting attacks.
-Specialist character allowing cheap BM removal / bonus to rally. Inspiring Walkers with penalty of BM when they are lost.

I'd love another look at that in a test game if you've got the time and see if it holds up still.