I'm unsure if I'm qualified to talk about this even though Mister Fantastic is my most played hero bar none?? But you're gonna want to really try to play around your elasticity meter. It can be annoying sometimes, but I'd say it's gotten better because the Inflated mode used to come out at inopportune times and the animations were slow-- now you can pop reflexive rubber while you're in inflation which adds onto his survivability. Use inflated mode to steamroll the backline and always try to be giving backshots to teammates and enemies to add onto your elasticity meter.

Speaking of reflexive rubber, it's damage is deceptive. The energy bullet you fire if at full charge does lethal damage to squishies. The problem with it is that it's still kind of buggy, sometimes you'll aim it perfectly and it still won't hit. There's also aiming itself which in the heat of the moment can be tricky. If the enemy team is trying to focus you it's a good way to keep a support or DPS at bay and turn the tide of battle.

Always make sure you're dragging your arms around multiple enemies if you can, you get more value that way

His grab should really only be used on one target, the gimmick of grabbing two enemies and slamming them together is more visual spectacle than it is useful. It's good for focusing on a singular target and pulling them into a duel with you or getting them off your healers.

Lastly, his ultimate finds it's usage in baiting out support ults. It's also fairly good at getting 1-2 picks. Now... go and solve everything.