It's commonly accepted in this fandom that humans have extremely fragile bodies in comparison to Eridians, but I think that's actually an oversimplification.
Because while Eridians are incredibly strong and tough on the outside, on the inside they are actually quite fragile:
They have no immune system, so anything that makes it past their enclosed carapace and internal heating will pretty much kill them
They have no natural defenses to radiation of any kind
In general they have very little organic matter so anything that targets their cells directly is quickly lethal
They are partially cold-blooded and live at a very narrow temperature range (about 20 degrees C)
They rely almost entirely on one, highly developed sense and are helpless if deafened (can't even remember the layout of a room)
They are completely helpless while asleep and can't control when that happens
They will forcibly become dormant after eating AND when badly hurt enough (their equivalent of going into shock)
They require more energy to function than humans do, and have very little organic matter to burn in the case of starvation
In contrast, humans:
Have an aggressive immune system and internal mechanisms for dealing with cell damage
Have adrenaline which allows them to temporarily ignore injuries and perform abnormal feats of strength
Are persistence hunters built for economy of movement and capable of extreme levels of endurance
Exist in a very wide range of habitats and on a diverse diet
Are very hardy in general, able to survive massive injuries, lack of sleep, prolonged starvation, and intense environmental conditions if given proper care
The quintessential example of this dichotomy between strength and endurance is the Going Fishing incident in the book: Rocky is able to survive and move in G forces that are killing Grace and to physically wrestle off the chair crushing him, but he collapses from his injuries almost immediately after. Despite being injured himself Grace then carries his 400lb friend up a ladder, is badly burned returning him to his atmosphere, and then proceeds to get some basic medical care, hype himself up on pain meds and keep working (albeit rather badly, lol) while Rocky forcibly sleeps.
The TL:DR is that Eridians are harder to damage, but easier to kill. They're like an rpg character with high armor and low health. I think Rocky would consider Grace to be very delicate at first, only to be blown away by how deceptively tough his friend can be.
