GrimCo
Hero
Being hit proof for one round is half the fight normally.
I'd say the issue is they are disproportionately good at upper levels compared to most other 1st level spells AND they allow for more nova potential in a low encounter days due to being off-turn actions. A 1st level Burning Hands is dog poop at 10th level, Silvery Barbs or Shield aren't.
Depending on how you or your DM designs encounters.
They are OKish at higher levels mostly cause you can't upcast them to gain benefit from those more powerfull slots, so they need to be viable at later levels. Burning hands on the other hand (pun not intended) can be upcast for more damage.
Since 5ed doesn't include magic items as mandatory, you'r tin can meat shield caps with his AC early on and it's what, 21 (full plate, shield, fighting style). Your eldricht knight can shield himself for AC26 for one round 3/4 times a day, depending on the level, reducing chance to get hit to 5% (and that one will hurt since it's crit).
At level 10, with AC21, monsters have around +8/+9 to hit. So they hit on 12-13, which is around 40-45% chance to get hit. Shield reduces it to 15-20% chance for 1 round. Silvery barbs is more useless, since DM rerolls and again it has 40-45% to hit you.
If you are not optimized for max AC, it gets worse, since chance to get hit rises up, so if monster hits you on 9-10 (55-50% chance to hit), Silvery barb isn't that good and Shield is better since it reduces chance to get hit by flat 25%. One thing that makes Silvery barbs better situationaly is if monster rolled nat 20 (aka crited you). Then it can turn crit into regular hit or maybe miss.