The most success that I've had has been using cruisers as the main line- three or four tyrants are my favorite- backed up by a light cruiser or two (if I'm bringing any carriers, then the light cruisers are the carriers) followed by like five escorts for fire support. I think Arks aren't mobile enough to really work because you've got to try to keep distance or they all come and board you to death.
I'm not very good at this but I do like the Mechanicus, getting a nova cannon onto a ship that you know just burned its mobility ability and watching it get blapped is quite satisfying. The disruption bomb is a good pick for them, knock out the shields first and then get a nova off on the hull, one-two punch. I like to focus out the admiral this way.
A faction that's weak to boarding in a boarding meta is always going to be up against the wall though. I also find I have a hard time getting through factions with a combination of good numbers + high health, like imperial Grand Cruiser fleets or Ork Battleships- there just aren't enough Nova charges to chunk through that collective health pool, and they'll outshoot you in the long term. I think some matchups you have to accept as probably losses from the outset, and just see how well you can do before they get you.
I really enjoy playing as Ad Mech from a lore standpoint but it seems right now the only viable way to win is with Nova Cannons, I have tried various set ups with mixed results. The most effective fleet is always the one with Nova cannons playing against slower moving Fleets. A fast moving Fleet like Eldar or Dark Eldar you have almost 0 chance unless your opponent is not paying much attention. No matter your setup, Taking some Escorts is a must to capture and hold the points while your larger ships try to stay at maximum range from the enemy and do high energy turns to drop Nova when available.
I would love to see more Ad mech like skills within the fleet to make them different from Imperial Navy, Right now there simply too similar. Maybe hull repair skills or stances to make the faction more unqiue.
AdMech are better in 2v2s when you have an ally that can help you keep enemies at range. Compared to Tau they just can't put as much damage on enemies as quickly so if the enemy is able to close on you then you'll be in trouble. @TheMadTypist definitely shares one of the best ways to play them in 1v1, using LCs as your Carriers while having some bigger ships for your "front line". Spotting early is very important so a couple/few Escorts to send up front to Scout is always smart. Abilities to help you spot enemies earlier are also a must have.
I love playing as AdMech whether 1v1 or 2v2 because I find them very fun to play but they are also difficult to do well with. As both @Cal89 and @TheMadTypist stated Nova Cannons are a must and that's why most of the ships have them.
Maximum cheese seems to be the way to go. Ark Mechanicus with 4 Dictators provides a lot of Nova firepower and vision+harassment with fighters and bombers. You got enough range to keep the enemy at bay for a while and can trade somewhat with more longer range factions.
The problem is as stated above when you face a more in your face faction or if something gets to close. Low troop value and not great DPS depending on your setup can spell death for your fleet.
Soften them up with Nova cannons and kite them as they come into range. Use escorts to scout them so you can pick at them from range. When they get close burn away and use abilities like the stasis bubble and the lightning orb to slow them down and finish them off. I mostly play 2v2 though not so sure about 1v1.