I wouldn't say that they aren't psychically adept, though they do have weak immune systems, which is the sole purpose they have the suits. Quarians lived in a relatively clean society before they were forced into the migrant fleet, and so because they had no need to develop a strong immune system on their home planet, they were forced into wearing the environmental suits and cybernetic implants to protect them.  Not to mention their DNA I believe is reverse of ours and the same as Turians, so they're limited on what they can consume or else it'll poison them. I'm not sure if it's DNA, but something along those lines. 

Anyway, this is why they generally have lots of procedures for people coming towards the migrant fleet about sterilization of germs, and usually have to eat a type of paste or heavily processed Turian food. 

Regardless, I believe it was Grunt who said or wrote somewhere that Quarians were hard to take down in a fist fight, which may suggest they're stronger than they appear, most likely due to their suits.  
Though, I also know that after a pilgrimage/during a pilgrimage, they can't start customizing their suits. I'd assume that a majority of the metal would go towards their fleets or research for countering the geth, at least up until the peace at Rannoch.