We aren't doing shields on UNSC vessels in war time, full stop.
That being said, the UNSC will suffer massive casualties, that's just how humans are at this point in time; to offset this they have a few research items that boost ship construction time and can get a very powerful economy rolling.
i do have one issue about saying this is a "war time game" the infinity simply did not exist at war time, it was post human/covenant war. at that point the UNSC would have started to use the forerunner technologies they recovered during the war and after that they found on ONYX.
with that said i believe that after you research the Infinity you should be able to research something that allows capital ships and the heavy prowler to gain shields. i say the heavy prowler because in the HALO book "The Thursday War" it is mentioned by parangosky that she wanted to apply the shield tech to the "UNSC Port Stanley" which is a heavy prowler.
obviously giving every UNSC ship a shield would give them an unfair advantage over the covenant due to the fact the UNSC has massive firepower and superior planetary defense.
to sum up my argument, i believe it is lore friendly that large UNSC ships and some select small ships get shields. this is based off of things mentioned in the HALO book series, mainly the Kilo-Five trilogy.
I can see that this a hot topic and I don't think that shields are a good idea the covies are meant to win they are technologically superior to humans but I do see the points that have been made. The weakness of the UNSC can make the games verses the covenant a mad house and I became couriuse about how much it would take to kill a covi so I did a test to check this.
I placed two reclaimer difficulty UNSC AI's against an easy COV AI. The reclaimers were on the same team and the fleet size totalled about 50 cap ships of varying levels but no lower than level 4 and a support fleet that numbered 150 ships easily so 200 ships in total. This is a fleet I would class as impressive and yet it was completely outmatched by a covenant fleet of 70 ships most of which were support ships, lower level cap ships and one freshly made titan.
I do not know the status of the military research of the two UNSC AI's as I did not check but with in the first volley of plasma torpedo's from the 50 cov ships left after the first MAC volley the 200 ship fleet went down to 110 ships.
The MACs then took out another 20 cov ships but the next torpedo volley halved the remaining 110 ships to 55 by now the cov support ships are mostly dead but the UNSC cap ships are down to about half there original number all having suffered heavy damage.
This is the only time I have ever seen an AI start to retreat and as we all know no one can run and fight at the same time so the battle became a massacre.
Out of the 200 ships that went in only 20 got out alive however all were severely damaged. This is how it should be and I hope it is how it remains.
The UNSC did eventually win by rushing the enemies capital planet but that was another 2 hours later with another 2 similar battles only difference being the UNSC did not retreat and had slightly lower numbers.
sorry for any spelling mistakes hope this is useful in some way.
The UNSC AI is far weaker than the Covenant's because the former's ships are based around alpha damage and being deployed in groups. This contrasts entirely with how the AI in Sins works where it is designed around protracted attrition warfare, meaning that it will send ships in piecemeal. Covenant ships, given their superior durability and DPS focused weaponry, will tend to function far better with how the AI operates.