They usually have 1 or 2 cheese to go for if their flaw can be mitigated or tolerated in exchange for their low dp and availability to mass them. Some good ones may come with superior op/dp, lots of caveats to make them work tho.
Common logistic flaw are large sensor profile from civ hull, bad overall fuel consumption and crew req for similar effectiveness, burn speed was also bummer but 0.95.1 gives industry skill that +2 burn for civs so escaping fleet ganging is easier.
and combat flaws are everywhere, bad flux, shield efficiency and narrow angle, mobility, armor and hull, system, allowing less hullmod per ship due to low op compares to same hull, you feel like over paying hullmods as each scales with less baseline performance(or larger hull for similar performance) and thus cant stack multiple hullmod to improve to godhood without lowering flux investment. They barely trade flux efficiently, being slow and heavy adding more risk to be ganged, and caught by same class or higher, and likely circle strafed by under class, results in hull damage.
Some common reasons of using them:
Expanded missile to max missile/dp, mitigate their inferior stats like flux, shield efficiency&angle, armor, hull mobility during flux trade confrintation. Kite, buffalo mk2, atlas mk2 are prime examples.
Maximaize fighter per dp, again taking the carrier back seat. Condor, any Converted hanger destoryer, colossus mk3. The latter 2 comes at a cost of worsen the fighters.
Artillery support, mudskipper mk2 and atlas mk2, one move fast enough the other is a capital shoot far and fast enough with autoloader system, with a lot of atlas they may generate crossfire zone ~1.5 km long, restricted by line of sight and need 100% leading range, another skilled officer or hullmod to fix that, some have good result with masses of prometheus mk2 and tac lances which feel nicer to kit with a few dp more.
Regular weapon mount dp density isnt very relavent as most ship failed to have enough op and flux to reach that barrier.
Punchbag like Venture, unusally thick armor packing missiles and missile system that can also generate insane counter attacks if piloted or instructed.
Other than that as dedicated ECM, nav provider, command center and avoid contact.
Logistic wise some can be shield your illegal cargo, provide raid fire support, surveying bonus, half-assed cargo and bed capacity.
Still many are still untouchable lost cause for me, these frigate shuttles as they have no future or usefulness to be massed and many of their civilian ai avoid combat until modified by militarized subsystem. And Gremlin, there is no hope.
good ones can have s-mod builtin as they can make this dp denser.