They are more powerful, but only when they have spell slots. Most games are low-level, where the Wizard only has a few spell slots. The balance of the game assumes that there will be multiple encounters in-between resting and regaining spells, so if you blow your load all in one encounter, you're going to relegated to firing a crossbow in the back.
As soon as your mage gets the Craft Wand feat, spell slots are meaningless. Until then, all you have to do is take an 8-hour break after every encounter.
That you think that is a good idea is not good advertisement for your games.
I am pretty sure
Nathaniel3W mentions that to criticize the power and versatility granted by wands, not to endorse it.
If it's the players who decide if they have 8 hours between encounters, you have a problem. I'd say in a typical adventure, for instance exploring a dungeon, players face 4-6 encounters.
And really, I don't get the problem with wands.
A level 8 wizard crafts a caster-level 8 wand. That's 18,000 gold (2/3rds of his recommended gear worth!) and 720 XP. He also spends 18 days at it. I don't know about other DM's, but when I run a game the players don't have 18 days to just fool around; if I give them weeks to spare because I'm running some kind of sandbox-like campaign, I sure will give them stuff to do with that time, namely explore or advance the plot, and probably punish them if they don't. Imagine the wizard decides to stay home and craft a fireball wand; in the meantime the rest of the party can play a whole adventure and earn a couple levels plus gain items, including maybe a wand!
So in most circumstances, crafting wands is an in-between-adventures thing, and the way I see it, it's mostly an NPC feat to explain how magic items exist in the world. Of course if you give the players millions of gold and they control the flow of time in your game, it can cause imbalances. But the problem ain't in the feat itself.
Not to mention in the end UMD is available to everyone, and with a 20DC any character can use any wand spending mere skill points, instead of a valuable feat.
And comeon, if somehow the players end up having a fireball wand you *know* the DM is gonna find a way to neutralize it in some encounters, somehow. Fire Mephits! and now goblin ninjas appearing around you! Surprise!
Also... don't you guys force your players' characters to sometimes skip sleep for days, or make them lose their gear for a while, or gets items stolen?
Sometimes I get the feeling people is running their games as if this was a cRPG, but this is the Gazebo, remember the DM is there to make sure the game remains fun and interesting for everyone. Your mage invests a feat and gold and XP to create a Grease wand? it's your job to make sure the utility he gets out of it more or less matches what he spent, if he figures out a way to "break" your campaign, then you fix it. Of course you must reward good ideas and sometimes a very creative spellcaster can solve a whole encounter, but if it happens all the time you need to work harder.