+35% shield HP sounds way better than 125 Evasion or a few points in Con (which I can't spare).
It's worse actually) Half of the attacks just don't care about shield, because even low frequency module in shield gives a very small amount of damage reduction for low impact attacks. So, any melee or crossbow guy, wouldn't even notice your shield, and with 3 Con + psionic health reduction, aimed shot form a crossbow will often mean instant death.
125 evasion can actually turn into a decent number with stuff like high quality tabi boots and infused siphoner leather armor, especially if you use drugs like Jumping Bean. Though even that would help only so much because a) without dodge, melee guys would still mess you up b) with your health and no decent armor someone sneezing near you can send you to reload screen.
Yep, this is unfortunate but true.
My knowledge of underrail is limited, but there are simply a lot of circumstances that you cant control. Like dialogue-initiated combat (no stealth opener, only initiative roll). Or high stealth melee enemies, stabbing you in the back unless you have REALLY good stealth and detection. You cant turn on your shield if you are incapacitated or dead.
A critical hit from an X-bow, like MirddinEmris mentions, or a grenade hitting you right in the body, means instant death if you have so low HPs, no armor or evasion. Stepping on a mine will also happen occationally even if your detection is decent.
I like to play as close to Ironman as possible, and get really annoyed when I die. Usually I could have avoided death, in retrospect, or I screw up by for example stepping in my own bear trap in the middle of a close shave combat.
But I try to make a character who can survive as well as possible. And the weak 3-STR, 3-CON, no evasion/dodge builds so popular here, always see the death screen eventually.
If I were to make a character with three CON, I would always have high AGI, stealth+evasion+dodge. Even this will not save you from psionics always, who cant miss.
If you are cool with the occational quickload, then you'll get by, I suppose.
If not, like me, then you got to plan your character not by "what can I do to my enemies when I have first strike", but "what happens when I get jumped". Being able to survive one round of incoming fire is the best, and that unfortunately means decent or high CON, and/or good armor. This limits your offensive capability, so you have to have a good plan. But this is just my personal preference.