My feelings about popular variants changes with my moods and the needs of the campaign but, overall, I simply don't care for them. Many seem invented to fill campaign specific slots while still others are a bit overpowered, taking emphasis in game-play away from cooperation and the party pooling its resources and talents.
Still, I do use them and here are my general guidelines:
Archetypes are the best at what they do. IMC fighters are the best class at dealing and absorbing damage, magic-users are the best class at casting spells and using magic, and so on. So, for example, if you invent a warrior class that exceeds the fighter class in some respect I will add in some disadvantages to off-set this skill.
You can't do it all. Table-top FRPGs are designed around groups of player-characters, each with their own set of skills, working together to solve problems. So your character will be very good at his profession but still have to rely on the others for certain things.
No evil characters. This is not a popular stance these days but I make no apologies. I don't find running a campaign with evil characters such as assassins*, anti-paladins, death masters, demons or half-demons, etc. to be fun. You don't have to be a caricature of the All American Hero or anything like that, but you won't be committing murder, rape, or the like in my game. I put a lot of work into running the campaign and I have to find it fun, too, or it isn't worth my time.
I won't offset a mechanical advantage with a role-playing disadvantage. To go back to my example of a variant fighter? If you are better at sword-fighting than your typical member of the fighter class I'll likely limit your armor wearing ability or perhaps decrease your hit points. What I won't do is limit your ability to have magic-users or clerics in the party because members of your profession "distrust" magic users. Role-playing disadvantages often turn into what my character would do types of arguments. No! Mechanical advantages will be balanced out with mechanical disadvantages.
THE BOTTOM LINE Would any reasonable player wish to play the variant class over the archetype? Does your Blade Master variant class dominate player-character classes, all but replacing stock fighters? Then, in all likelihood, the class is over-powered for the purposes of my campaign. Of course, it should go without saying if the class is meant to replace an archetype (something I've never done) then this is not a consideration.
* Yes, I have assassins in my campaign but they are NPCs, very Lawful, and have a lot of laws and societal expectations built into the specifics of how they are used.