@PentaKon The answers explaining why UMD won't be OP hold in the context of normal campaign, but I have the anecdote a Thief being OP in a west marches campaign. The homebrew downtime system we had made it easy to create spell scrolls at half price so with UMD the Thief's "spell list" approximated every spell known by every PC spellcaster in the campaign.

Optimal UMD plays included: abusing your "spell list" with long term concentrationless buffs (death ward, find familiar, find major steed, etc.); abusing your "spell list" for unmatched out-of-combat versatility; using a good concentration buff every combat;

sneak attacking with the Action and then casting a spell scroll with the bonus action; casting green-flame/booming blade to sneak attack insted of using the Attack Action; casting an AoE spell scroll with an action if it was better than sneak attacking.

With the ammount of gold being thrown around in that campaign, a Thief could have far more versatility than an Arcane Trickster, could more or less match the number of spells cast, and could occassionally splurge gold on a higher level spell that an Arcane Trickster could only dream of.

Compared to a full spellcaster the Thief had more spell versatility on top of all the Rogue options, but couldn't afford to cast as many spells (especially high level ones). This would have been very strong, but not OP compared to all the other stuff going on in that campaign: good magic items were aplenty, normal spellcasters could also use the abundance of spell scrolls, etc.

The real limiting factor was the action economy, even in such favorable conditions the Thief couldn't cast a strong spell scroll with their Action and also sneak attack on the same turn, so for a while the UMD Thief remained a theoretical meme between our friends...

until one of my friend got a Scimitar of Speed (great item for Arcane Tricksters too btw), and then boom! everything fell into place because now the Thief could cast a spell scroll with their Action and sneak attack as a bonus action in the same turn. The ultimate gish in 5e.

There was tons of good flavor tacked on too: he was called Linku; the Scimitar of Speed was obiously a Master Sword; he also had a belt of giant strength reflavored as the golden gauntlets; the familiar and greater steed were the fairy and the train from LoZ Spirit Tracks; the rest of the scrolls generally covered for the variety of other blessings, magic, and equipment that Links find in their games.

That's all just to say that the OP thief you imagined really did exist at least once, it just won't be allowed to exist in any reasonable campaign.