So, up to now, I find that Mini Fortress are more like super engineer in my defence line than, a fortress. Mini Fortress does deserve to be under the tab "Support" since its firepower is just... Nothing. However, it is much more reliable at fixing a huge cluster of turrets than those FRD suiciding engineers, which doesn't fix anything but run to death when a CPA suddenly emerges from the wormhole.
Interestingly, MFs are not quite targeted by anything from the AI (probably because the Spider Turrets stalled the Bombers), so it just sits there fixing >100 turrets while doing some moderate damage.
Then, I tested Fortress Beachheading, on a heavily fortified Turtle 6 MkIII world with a whole bunch of special forces. I bring in my special force too: A team of 20 MkII Engrs and 4 Riot Controls, 1 Flagship, 1 Leech, plus 40 Mini Rams (for taking down starships, but I forgot to use them... which caused my expensive special force to die. RIP, starships).
Fortress is interestingly not targeted in construction. But I still think it takes too long to build, even with 20 MkII Engrs. It took ~30s, if I remember correctly. Then I built Spider IIs, Missile IIs, Laser IIs, and soon enough there is no more ship floating around. I almost lost the force field, though.
Side note: It seems that Auto Kite always kills my ships in beachheading. Is there any way to let them move back after they kite away? Not quite like Attack Move since I don't want them to chase enemies.