The Doctrine isn't the best doctrine. It is good but it has clear weakness and if we were going on application of a doctrine then Ultramarines would take that in spades, clubs, diamonds and even hearts. Their doctrine allows you to move and fire heavy weapons at full effect, no penalty along with allowing you to use double tap bolter for free at that point WITH a bonus AP1.
In regards to combos, Captain Dakka says hi.
Their relics certainly can make for a stiff captain smash but something people seem to keep forgetting is how to play all these new toys. So lets begin.
Iron Hands are slow. Shut up about being able to move and shoot heavy weapons for free on turn 1, if you are going gun heavy you ain't moving turn 1 and don't even try and kid on about it. If you are moving its because the opponent ducked out of LoS during deployment and just stole your turn 1 shooting. They aren't going to be moving far nor fast and if they are it will be in the flimsy land speeders. Not hard to remove (since they will lack any repair support nor have the wound count to rely on). Also I want to add that moving to shimmy over to get better LoS is rarely effective when you only have 6" move or can't ignore having to go through walls since your a tank! If you are moving FORWARD then I have to ask some serious questions because...I know we joke about driving closer so we can hit them with our swords but...at least then we are talking about swords...not the butt of our lascannons and heavy bolters!
They are fairly predictable too in that regard as you know the moment you hear Iron Hands you can be assured they want to gunline and shoot. The main thing to remember is if you are in a shooting contest with them, you are on even grounds if you have a captain backing you so no worries, not like templates are a thing anymore. People seem to think the auras we have are tiny...6" is miles if you actually position better, the main reason for castling tight is more to do with getting in the terrain piece, not auras (and EMPEROR DAMNIT Gregor...BALANCE ON THAT TOP FLOOR OR OFF TO THE CHAPLAIN FOR YOU). The other major thing to remember is they will also have a harder time bringing chaff clearing tech without specialising if they go in on stalker bolt rifles. Not going to lie, the all heavy weapon list I made was quite something to look at for myself.
Another aspect of their combos and repairs is remember their techmarines need to glue themselves to the side of the tank to keep the repairs going and the ironstone is only 3" in aura (which for something that only works on tanks can really hamper formations of armour around, you try!) and on top of that, you can't repair whats dead. If you think 6+++ is going to save you, don't count on it. When things get down to the wire for killing then sure but most times, you find that the tank is dead with change to spare. To cap that off, tanks ain't cheap and the killer of them is charging them, instantly shut down and most tanks don't have the gun to stop charges.
What I think will become a big deal with Centurions again, not tanks. The big thing Grav-turions have issues with is being charged and deep strikers do that, the case I bring here is it won't be tanks or dreadnoughts we are scared of, but centurions.
As it stands however there are two camps. There is the Iron Hand version which will without doubt be the most terrifying thing to charge on the emperor graced universe (5-6 gravturions overwatching on 4+? Na, I don't think I will charge that!) but similarly the Ultramarine version does have "for the greater good" (sorry...what was it? Tau focus...no...Defensive Blueberries? Meh, close enough) which if there is even a second squad of centurions nearby it will equally remove a threat charging.
To be honest, I am torn. There are some serious considerations in favour of the Iron Hands and I am torn because I LOVE the look of the codex but I am being cautious because to be honest, this just REEKS of hype and overglorifying effects and abilities.
As I like to Say:
"In Theory, Theory and Practice are the same. In Practice, Theory and Practice aren't the same"
Lots of theories like WW1 generals assuming what the battles will be like without actually having any proper knowledge of what it will shape up like. After all, there is the Eldar Flyer Spam list out there just laughing at shooting (sure you have re-roll 1s but you are hitting on 5s) and Tau Riptide lists are still a thing and while yea, you can negate the HBC+ATS with the Ironstone for dreads and tanks but there will still be fusion blasters to contend with in some lists and you still need to deal with the drones and they are just a mobile because those Riptides move 14" which is nuts.
Against the mirror it will be a game of cat and mouse with the winner being decided by a mixture of skill and luck.
Heck, you may even face a bunch of Whirlwinds and Thunderfire cannons from one guy and just say "can't hit what you can't see"
Have a run in with the classic Shadowsword + 3 dawn shields on captain bike jet eagles and now you have that to contend with.
Maybe you run into a grey knight player and you concede to them to be nice as so they get a win for once.
After having a little vent earlier in this post, What I want to say is a lot of people are calling "OP" without actual testing being done proper and thorough. We need to see the Iron Hands actually have some real live fire games against what the world has to offer and lets see how far they get before we scream OP.
Unless its a new Eldar codex...then we just assume its OP because eldar...