You have two opposing forces, the Cardassians, who fare very poorly left to their own devices, and the Jem'Hadar, who completely lack self sufficiency. The Breen are the odd man out, self reliant and effective enough to make a run for it on their own, but with little to gain from their allies. Cardassians are unreliable, actually able to turn on their masters under certain circumstances. They need insurance to be safely used. They are however the only option for a rapid, and viable response to an incursion.
Dominion Elites are slow. Hugely powerful, but slow. Jem'Hadar frigates are fast, but underperform in a terrible way. The cure for this is in the Elites, which have risky, but highly rewarding influence over the Jem'Hadar. A high level fleet can be a fearsome force , but the loss of a single ship can have catastrophic consequences as well. The Jem'Hadar have a bad habit of suiciding if they fail their leaders. They are also highly incapable when it comes to repairing on the go. Returning to their fortifications, or building them up as they go, is the only safe way to do battle. If a battle leaves you vulnerable to counter attacks, it's time to make as swift an exit as your lumbering behemoths will allow.
On the plus side, your fortifications are spectacularly effective. Your OWP's can be centrally powered to be greatly improved, and your trade station is both a battle station and a point for resupply. Unfortunately, if you choose to forgo the Cardassians because they are weak, you may find your defenses being the only thing you can defend with, as the Jem'Hadar will never perform without their masters.
Jem'Hadar frigates are weak, ineffective ships. They underperform everyone else, even when upgraded with Breen weapons, they're still not worth the cost on their own, but they garner a massive level of stat boosting from the elites.
A high level Leviathan will get you 12% weapon cool down, 10% chance to hit, 20% angular thrust, and 20% weapon range. Non Jem'Hadar ships only gain 7% weapon cool down and 10% weapon range. Most of this is from the level 6 ability, Enforced Loyalty is more for the secondary effect of preventing mutiny from Cardassian ships. These are both passives.
A Devastator with Glory To The Founders, also a passive, maxed out will get you 5% weapon cool down and damage, per nearby Jem'Hadar, to a maximum of 10. Effectively a 50% increase to both weapon cool down and damage in any decent sized fleet. This is only for Jem'Hadar frigates.
An Enslaver with Victory Is Life does even more, but this one is a a 30 second active. 100% weapon cool down and damage to Jem'Hadar frigates.
For thirty seconds, a swarm of Jem'Hadar will outperform the Borg. The risk in this is that the Leviathan and Enslaver ultimate’s both come with a caveat, lose the ship and your Jem'Hadar will suicide.
The Keldon, likewise, is important for overall fleet power, with a 10% weapon cool down and shield mitigation boost for the fleet, and further Cardassian only improvements.
The trick to becoming a nightmare with the Dominion is getting those high level ships to support the swarm of frigates. To this end, you have a very strong defense.
Your trade ports take 8 logistics, restore shields and antimatter on nearby ships, and are otherwise a major defensive installation with durability and damage on par with a level 1 star base.
Your weapon platforms, while initially terrible, are easily upgraded by utilizing central powering. Two Will of the Founders, with Power Distribution Networking researched, will give your platforms an extra 2500 shield points, a 40 point restore rate, 1000% plasma torpedo cool down, 200% energy weapon cool down, and 40% weapon range. They require large fleets to drop, and are severely damaging.
The Dominion are designed for a slow, exceptionally well-defended expansion, building up fortifications and an economy to protect and restore your fleet as you go. You can build a powerful defensive position at the same time you're creating your economic infrastructure.