Just keep it under control. I allowed +5 UMD gloves as the max. If you start going +10 or +15, then the artificer/rogue has nearly zero chance of failure.
You can get Spellsight Spectacles for 2.5K. They give you a +5 bonus to UMD checks to activate scrolls, where is you have your real chance of failure anyways (wands are easy).
I let his character live even though he should have fallen off a wobbly grell-infested platform that you and I know about. Hell I even had evil priests raise him and give him a nice demonic addition after he was killed several months later. And I tolerated his munchkin monk/ranger/wizard/fleshwarper. I'm super generous.
And he still won't let me add a charisma enhancement bonus to a hat/circlet.