To begin with shields are useless past level 10. Past that, against ANY build your shield will struggle to cope no matter how high you put your heat/energy.
I ran a test yesterday and with 200e/113 regen my shield was depleted in two turns against a heat mech (energy shield). Regardless to this I still did some calculations to see if it was even worth doing.
To get 200e,113 regen it took up 5 module slots leaving one slot for health
For a common mech, their regen only takes up 2 slots allowing 4 more slots for health.
The above print screen shows that EVEN if I keep my shield up for more than two turns, I will still lose a duel with equal damage on both sides. That being said it's almost impossible to keep my shield up for so long.
To make matters worse in using this shield and the required modules, my mech weight goes up to 647. The other mechs weight is only 614 meaning the likelihood of me even having stronger weapons is unlikely.
The shields are MASSIVELY outbalanced with the game, which is a shame as balancing them would make the game so much more interesting.