Unbalanced for what? Balance is complivated and has different points depending on what is important.
General bands of resource generation potential in the course of the early/early-mid-game, when individual planets matter most.
Number of planets in your core area is already one of the biggest indicators of power by year 75, which is the point by which most games are effectively decided as power snowballs start consuming other empires. Expanding the power of planets via interesting modifiers makes this more unabalanced, not less, as people with more planets will also have a better chance to draw better modifiers, further expanding their resource/power base.
Negative modifiers aren't really a balance, because to be 'interesting' they tend to be circumventable, and thus just affect prioritization rather than lowering output. IE, a world with a farm-malus is not going to lower empire food, because you just use it as a non-farm world, but a world will a interesting mining bonus will always be prioritized as a mining world (unless there is something even better for it).
Also those unbalances lead to interesting decisions, something that is desired in a game.
I disagree this is the case in this context of planet modifiers. An interesting decision needs to weigh a cost-benefit, but strong modifiers break the link to make it a cost
or benefit, for which people always choose benefits. If a world has a +20% miner benefit, there's not really a cost-benefit, there's a benefit or an opportunity cost of not taking the benefit. If I'm short on minerals, this lets me make up the minerals. If I'd be able to scale up alloy production with more minerals, the lets me make use of the minerals. If I wouldn't increase my mineral consumption, taking the minerals would let me free up another less-efficient mining world to more efficient purposes.
Stellaris is, at heart, an economy game geared around pursuing economic efficiency. Any modifier that affects efficiency is going to see the decision tend towards whatever is efficient or- if not strictly efficient- useful and effective, which is a different form of efficiency.
If the planet modifier is
not making an impact on output or efficiency, what, exactly, is it modifying? The color? Flavor text?