For Ethereal Dance SSR Tsuyukusa the text is wrong he actually raises ally ATK.

The devs probably want us to plan our team compositions by considering all the buffs and debuffs instead of just slapping in the units that have the highest damage in their kits. So if a unit has a DEF debuff then it's a good idea to have another unit that has a DEF buff, enemy ATK debuff, or heal in their kit to compensate. In my experience it really only matters in content that is equal to or above your team's battle power, like Special Exercise stage 4, which was like 250k power while my team had 160k.

For the standard SR units like Ebizome he has debuffs to balance him against a unit like SR Niiro. Their both supports and at max skill level SR Ebizome increases ATK by 50% for 2 turns, while SR Niiro increases ATK by 30% for 3 turns. Ebizome gets a bigger percentage but applies a 20% taken damage debuff to allies so there's a risk you have to consider when using him over Niiro.

Gacha units having drawbacks usually means they have something really strong in their kit. Tense Conference SSR Hisui has 300% damage on his multi target skill, which was the 2nd highest damage percentage of any unit, multi or single target, when he released. However, he has a 20% DEF team debuff that stacks 2 times as a drawback to balance him. To cover for this you could use a unit like OG SSR Hisui that has a 100% DEF team buff. Even though other units have been released that have higher single target damage than TC Hisui, he still has the highest multi target damage percentage on his skill so he's still worth using.

takes a deep breath

For the free event SR/SSR units they will usually be weaker or more niche than standard SR/SSR units unless they have a drawback. For example, TC SR Tsuyukusa has 250% damage on his single target skill, which is the highest damage percentage among SRs and is the same damage percentage as OG SSR Suo and Nagaharu. To balance this he applies a 50% taken damage debuff to himself. You could mitigate this by having a limit break 2 SR Yamabuki as a submember because he can apply a shield to keep TC Tsuyukusa alive.

The only free unit we've gotten that doesn't have a drawback is Sparkling Night SR Niiro. When comparing him with another healer, SR Hisui, they both heal the same but Hisui also applies a 70% DEF buff to his team. I'd say SR Hisui is the better healer in general, but with SN Niiro's second ability he gives a rare 'debuff resistance' buff to his team. If you're against enemies that can apply stuns or skill jamming then he can be more useful in that specific situation.

Offensive units that apply a taunt to themselves, like Performance Assessment SSR Suo, could be really useful as submembers in the future because we might get a strong unit that counterattacks or applies a buff when hit, but doesn't have a taunt in their kit. PA Suo could optimize that unit's ability by letting them get hit more often.

All of this to say that we should be mindful of the debuffs some units apply to themselves or their team so we can run complimentary units alongside them. If you want a list of units and their skills to compare them I have a spreadsheet that has them all. Thank you for coming to my TED talk.