I really don't see how they're underpowered. Really, few antagonists that aren't named major NPCs should be attacking from more than 3, so a 4-shift block on its own is pretty effective against most things. If a caster is equipped properly, they should be able to pull off 8-10 shift shields fairly easily.
I agree, they are absolutely NOT underpowered. Many wizards will have a 4 or 5 in Conviction. Many also have a shield bracelet (or an equivalent) that would give a +1. Finally, many wizards spent an enchanted item slot for something like Harry's Duster. Now let me show you how this would play out...
Harry enters a room to find there are guys with guns. He instantly puts up his bracelet with a 6 Power (5 for strength, 1 for 1 extra exchange). Until his next turn, every enemy has to get a 5 to hit him. Keep in mind that 5 is something that is so good, many campaigns don't allow a skill that high. A thug would have maybe a 2 or 3 at guns. A trained cop would probably have a 4. Only the best would have higher. All this is for one mental stress. Even if one gets through, you can use your duster as armor. Your block would cancel out most of the attack, and most weapons are 2 or 3, so the duster would stop most or all of it.
Before I continue, compare having to roll Athletics. Yes, you might get an awesome roll but you are just as likely to get a -2 as you are +2 and how screwed would you be?
Next turn you decide to spend one more casting action to up the duration. Because you have the shield bracelet you can get it for 6 more exchanges. Now you've taken your second stress. (If you had spend an additional stress to make it even higher in the beginning, it would be nearly indestructible.)
Now you have the freedom to move as you please. You can even turn it into an Armor of 2 if you think your block might get canceled (though it then won't stack with your duster). With 6 more exchanges, you have options while they poo themselves because their bullets can't hurt you.
Here's the final kicker... you can use that stored shield energy for another spell. (The rules are on p.260) As long as there is one exchange left it is never a complete waste.
This isn't even including your ability to protect EVERYBODY. I think it's one more power required to protect a person next to you or 2 for a zone. In a group fight, you can put up an area block, saving everybody, then they can attack without fear. If it counts as armor, they can even dodge while getting your bonus.
The final factor you need to consider is how incredibly overpowered wizards are. (Which is ok. I don't mind, my players don't mind) Evocation gives you this boost to your defense that you wouldn't already have. It also gives you an incredibly powerful damaging attack. How many of the other players will have a Weapon Strength: 5 (or higher) attack? Fine, it costs stress that goes away pretty quickly. If you have a 5 in Conviction you'll even have an additional mild mental consequence.
If you throw out Evocation like a thug then, yes, it can be underpowered. Maybe you're facing that creature that is incredibly resistant to magic and is coming towards you to crush everything. Boost that Fuego up and attack the wooden floor, making the creature fall to the lower floor. You can't really do that with gun or sword.
Maybe the entire room is filled with vampires. Heck with it, make it a zone attack and burn EVERYTHING. Conviction:5 means 5 damage to everybody in the room if they don't get a 6 on their dodge. (More damage if they get anything less than a 5.) That only costs ONE STRESS.