Unearthed Arcana vs. Published Rules
A bit of a frame challenge here... Trying to compare UA rules to officially published rules is going to give you nothing but headaches. As the UA article itself says:
The material here is presented for playtesting and to spark your imagination. These game mechanics are in draft form, usable in your D&D campaign but not refined by final design and editing. They aren’t officially part of the game and aren’t permitted in D&D Adventurers League events.
If we decide to make this material official, it will be refined based on your feedback, and then it will appear in a D&D book.
To put it another way, the fact that the UA Sidekick rules are problematic is something the developers realized, so they rewrote them for publication.
In the published version, your question is moot because the Sidekick rules cannot be bolted on any old statblock. There are four specific statblocks - Warrior, Expert, and Spellcaster - Mage, and Spellcaster - Healer.
The advancement tables don't even appear in the same place.
To reduce the load on both players (when used in the single-player/single-DM option) and the DM, Sidekicks have drastically simplified advancement. They have very few choices on leveling up - ASIs are predetermined, Spells are predetermined, they don't get feats, and so on.
I realize many of these links may not work for people who don't actually own the content on D&D Beyond, but that doesn't have any bearing on the statements made in the answer.