[Spoilers. Most reading will have played Reach by now but just in case.]
I've always been wary when it comes to (near-)voiceless protagonists in story-driven games for a number of reasons.
The ability to design your own character is all well and good but if they never or rarely speak then it's hard to put yourself in their shoes.
If I was a member of Noble Team I would talk to the other Spartans about what's going on, try to comfort frightened civilians, engage in the (albeit limited) banter, etc.
Given how 6 has just enough dialogue across the entire campaign to fill a single page header, I invariably end up feeling like a mute or a robot that's trying to figure out human emotions; 6 doesn't show joy in victory, fear when Reach is glassed, or sadness when his/her teammates die.
Many people encourage you to fill in the blanks yourself but if the other characters don't respond to what you say (which they can't) it begins to sound like a poorly-written self-insert fanfic, for lack of a better term.
On top of that I rarely felt like an integral part of the team when playing; the lack of dialogue never elevates 6 above 'Some guy who's just here to pad the numbers' to me.
You start the game out as a stranger with a mysterious unexplained backstory, and end the game as the same stranger with a mysterious unexplained backstory.
Your teammates do warm up to you over the course of the game but it's hard to see why when you barely talk to them, and their deaths would've packed a much bigger punch if I'd considered them friends.
I do like the customizable armour though, I just think it would've been better if there was an actual voiced character under it all.