If planets do have hazardous there will be story and gameplay segregation, lore wise though Hanar will only have seconds to live on very high temperature planets. Their anatomy prevents them from being solders in any form since it'll just hinder the others that it fights along side with which could spell their deaths.
Hanar have more resilience than a few seconds. They might be more susceptible high heat, but one is not going to instantly burst into flames, or deflate like a balloon the moment high temperatures hit it. If the atmosphere is hot enough that a Hanar is going to burst into flames than the planet in question will spell instant death for humans as well.
The galaxy's level of environmental protection seems to be pretty much the same across the board; a human's, an Asari's, a Turian's, a Krogan's, and a Hanar's space suit is going to offer roughly (to the point of it being negligible) the same level of protection from adverse conditions. If the suit is compromised/breached, then yes a Hanar would have a harder time of it compared to a human, but outside of narrative fiat our character's suits are never breached or damaged; even when pierced by high velocity rounds.
As far as combat prowess is concerned, I agree that the Hanar would be low on the 'likely to happen' list; truth be told I would much rather have a combat capable squad mate in the form of a Rachni or Elcor; but it wouldn't be because the Hanar is incapable of engaging in combat or operating in hazardous locations.
Even though it would be unlikely to see a Hanar in combat, I would argue that they still would make capable snipers and/or support operatives. We all like to laugh at Blasto trying to dual wield a pair of Carnifexes, but those are weapons designed around humanoid ergonomics. Commander Shepard would look equally ridiculous trying to operate Hanar designed weapons, so faulting them on the grounds of: "They can't hold a gun" holds no merit.
- Their thin profile will make them hard to hit from range compared to a human or Krogan.
- Their multiple limbs can operate independently of each other. We all like to envision an octo-wielding maniac, but a Hanar hacker? Combat Medic?
- The fact that they hover makes traveling across rough terrain easy, and anyone attempting to track them will be hard pressed to follow something that leaves no tracks.
Granted, a Hanar would make for a horrible frontline soldier compared to your Turians or Krogan, but as a supportive operative; like the Volus in ME 3 MP; they could offer some surprising advantages.