you can't compare the cube to the tavara. the tavara has a very impressive firepower cause it has a very special energy source (i won't tell you more about it, cause it will be part of the singel player storyline).
the cube is a regulat battleship and the tavara is the final romulan dreadnought. you may compare a fusioncube to the tavara and "bang", the cube fires one torpedo and the shields of the tavara are gone with the wind.
the cube has large technology requirements, that's for all borg vessels. therefore the borg have a giant ammount of crew on each vessel and thanks to incubation complex they don't even have to worry about crew! which means they lack a whole resource which other players have to care about.
other way the borg can assimilate other vessels quite easy (holding beam on small vessels like rhienn for example) and then get vessels FOR NO COSTS and the enemy LOOSES THE VESSEL, which is much better then destroying them.
The D'deridex has very low technology requirements, that's part of the romulan balacing system.
and again: in flops not every race is superb in every sector. the races are very specialised and you will have to chose the race which fit's to youhr strategy. if you like playing with battleships you can use the romulans, but then don't cry if i beat you up with borg assimilators and scoutcubes. if you like to use small vesselsyou can use the federation, but then don't cry if i beat you up with borg cubes.. or assimilate you?...
there are nearly no allround vessels in flops, discover the strength and weaknesses of each ship

and finaly, there is a new balacing system for beta3, but it's not likely that there will be technology requirement changes. these will come in beta4 when every technology tree get redone cause there will be a new feature (a new kind of vessels).