Well... it kinda comes up to situation. mewtwo's offstage game is good with his great aerial keep away game & fantastic aerial drift, but rather risky due to his high DJ & other characters high chance of beating out his edgeguard like cloud or mario with counter attack bairs & nairs to keep mewtwo on the defensive if it goes for a full on approach as a edgeguard, so dair offstage is rather unreliable in my opinion. Phase dair isn't reliable do me just because they need to be completely below mewtwo in order for it to spike, especially because mewtwo's dair only has three active frames that it will spike, & you really are only able to go for one dair offstage as a safe option, as dair to DJ dair is really unsafe & since by the time you rise up again they can most phantom footstool off mewtwo's rising attack (other than up-air but they can airdodge that into a sweetspot up-b like lucina & even possibly stage spike you killing you) or just land on stage with a DJ at the right height or just sweetspot or just attack mewtwo when he rises no matter in he AD's or not because most high tiers can delay attacks in this situation to hit mewtwo, you get what I mean. So in short summary, random dairs as a edgeguard are bad, against good players as some bad players will fall for it
On the other hand, following drifts after setups & attacks you throw out like d-tilt or fair, or most notably airdodge reads are pretty consistent. Since other thank mewtwo lucas & ness can't change their aerial drift mid-airdodge, if you read a airdodge into dair or condition them into a laggy or poor option dair offstage can become very good if used right. Like, alot of mid level mewtwo's opt for a 30-40% d-tilt into a dair spike on the edge because the opponent thinks mewtwo will go for a attack & panic AD instantly. Out of nair dair is really bad on even half decent DI, but some other moves are much better, as for setting it up, you'll need to experiment with this concept yourself because I can't be bothered to explain myself. So AD reads into dair work great if your good enough to recognize the opportunity. Attacks by the opponent work the same just remember which attack they'll throw out & adapt & space yourself accordingly. And as a combo finisher really dair isn't that good at all, infact it's rather bad on good opponents. It's made out to be one by most mid-level players but there are barely any good true dair spike setups that work consistently. So I wouldn't even recommend it at at all if you wanna use dair right. You'll see this alot in mewtwo montages, but don't be fooled.
Other than that SH dair like he said can cover 2-frame grabs pretty well & trades great with good timing. However the opponent can tech it by DI'ing in & teching if it is a tall wall stage, like omega wily castle. It does require specific practice on each character to get the timing right, but if practiced enough it becomes really good on some characters like olimar & rosalina & kirby. So i'd recommend the 2-frame the most just cause of how consistent it is, then next the read.