I've leveled one character almost exclusively from 2 to 13 so far, mostly running solo short or very short missions with custom enemies.
He is a fire/kin so is squishy, but a controller so that takes some of the edge off.
Thus far he hasn't had TO much of a problem most of the time, but it may be because I've yet to run across one of the 'armor' set custom NPCs, who supposedly have pretty nasty mez resistances.
Yes, I would imagine a controller wouldn't have too much trouble in mine until getting to the bosses which do have armor and that usually means player-standard mez protection.
I noticed when I soloed them with my blaster that as long as my immobilize got them I was pretty safe. Despite having various blaster powers the melee types never did use them. But I notice the trolls and skulls did the same thing so I don't think it is a problem with my customs.
That's good to know someone has gotten to 13 fighting custom mobs. I've started a new character on Protector that I'm going to get up to 5th so I have someone to test with.
I've heard a lot about EB's and AV's too. But I assume anyone who doesn't think they can handle those just needs to run at Heroic. Even with my characters at the level cap they still spawned as Boss and EB's respectively. They were tough but not unbeatable.
*sigh* I think I need a break from this. The frustration level is too high.
I made an arc with custom critters for low level as well.
I think the issue is that even at the lowest settings, custom critters have 2 attacks, and if you take no armor you're taking another attack set. So they get more.
My intent was to provide something for the solo lower level defender/controller who might have a limited number of attacks to be able to do. I tested it with a Level 1 TA/Archer defender. Snap shot had the lowest per attack damage, no defenses or heals.
Turned out a bit rougher than desired, but I could get through. If I rush, I can get killed, at least at Lvl 1. Tried it on my L34 Emp/Dark, and its a breeze.
I wanted the allies to be right in front, but they wind up a bit inside anyhow even on 'front'. I had trouble finding a map set that worked right and let you get to the allies easy enough.
Also wanted an Lt as the boss encounter, wound up having to split him into a second group type to keep the all-minion spawns.
Selecting 'standard' 'hard' 'extreme' various which armor powers are active. Not all the armor sets have mez protection at the same level.
I think you are right. I really really wish we could better customize our custom powers especially having a few NPC sets with a minor ranged or melee attack paired with a minor defence or resistence that gives standard mez protection.
Anyway, my SG was kind enough to run my story last night and I feel a lot better about my customs (and my story in general). Everyone seemed to have a good time even when we team wiped once (accidentally aggroed a 2nd group).
My AV (who spawned as an EB because we were on Heroic) went down pretty quick and he has Fiery Aura at Hard. But then if I had 8 people pounding on me I'd probably go down under the barrage too. I know when I ran it solo the final EB was a very exciting challenge so I'm going to leave him alone.
Thanks for the insights and comments. 
I created a small custom mob. Most have only attacks, a range and a melee, no armor. I tried to make them beatable while still a challenge. I put in some skulls to the mission to make it scale to level 14 and some trolls to make the minimum 5.
I know the mission can be done at level 14 by both a blaster and a scrapper. The custom guys are rough but I liked the challenge.
My wife saw the mission was 5-14 so she went in with her level 6 scrapper. She got killed 3 times and finally gave up promising to try again at 10th.
Is it my imagination or do the custom mobs NOT scale down very well? Do they have all their powers regardless of level and just the damage scales down? Is there no other throttle on those guys?
Just curious what those playing the MA at the very low levels have experienced. I'm not sure what I can do to make my arc actually work for the full range. The only thing I can think of is to not use custom critters at all.