The Borg, especially as depicted earlier on in the franchise (throwing out the VOY tendency to have them be a species you can "make a deal with" for example), would annihilate the Dominion.
Everybody gets one. This is yours. Can liquids be assimilated? Borg tech might not work on the Founders. The Jem'Hadar and the Vorta wouldn't last though.
Once they take care of the Vorta and Jem'Hadar, they can just burn the Founder homeworld to the ground.
Can the Founders imitate complex technology? Odo made clothes, a coffee cup and the illusion of filling it up. Could the Founders infiltrate a Borg cube and cause a ruckus?
I'm sure they could to some degree. Though there are thousands of cubes and smaller craft. I'm not sure they could create the same kind of ruckus with the Borg as they did with Earth. I don't think fear of invasion is something that would work with the Borg.
The DS9 I just watched, when they were drilling for a Dominion invasion by hunting Odo, he said "They can be anything." I'm not sure how far that reaches.
The Jem'hadar's genetically induced chemical dependencies would likely thwart the Borg, who depend on assimilating their opponents to win. Advantage Dominion.
Not just clothes and coffee mugs... Odo's fully functional communicator must have been part of him, since it went away whenever he shape-shifted. Kor
Odo said if you scan him while he's a rock, he reads as a rock. That wouldn't be true if there was a Starfleet communicator inside him.