Why it is worse, mechanically
UNEVEN HEATING. A microwave heats water directly and unevenly, producing hot and cool regions within the same mug. A kettle heats from the bottom, which sets up convection and mixes the water as it goes. Tea brewed in a microwave is brewed at a range of temperatures at once, and the result is partly over-extracted and partly under-extracted. NO VISIBLE BOIL. You cannot see when it is ready, so you are guessing, and mugs, volumes and ovens all differ. With a kettle, boiling is self-evident and repeatable. NO POUR. Pouring water onto leaf agitates it, which is a real part of extraction. Water heated around a bag that is already sitting in it gets much less movement. The result is not undrinkable. It is less consistent, and consistency is most of what makes a brewing method good.
The hazard: superheating
Water in a very smooth container, heated in a microwave without disturbance, can pass its boiling point without forming bubbles, because bubble formation needs nucleation sites and a clean smooth mug may not offer any. That water then boils violently when something disturbs it — a spoon going in, a bag being dropped, or the mug being lifted — and can erupt out of the container. Burns from this are documented and it is the one genuinely risky thing about the practice. It is easily managed. Put something in the mug BEFORE heating — a wooden stir stick, the tea bag itself, or a non-metallic spoon — so there are nucleation sites. Use a mug with some surface texture rather than a perfectly smooth new one. Do not heat plain water far longer than needed. And let it stand a moment before moving it.
If you are going to do it, do it in this order
Put the water in the mug with the tea bag or an infuser already in it. That gives nucleation sites and means the leaf is present as the water heats, which at least brews it evenly as the temperature rises. Heat in shorter bursts and stir between them rather than one long run. Stir at the end. This is the single biggest improvement available and it costs nothing: it evens out the temperature gradient and agitates the leaf. For green tea, stop well before boiling. The absence of a visible boil cuts both ways — it is harder to know when you have overshot, and green tea is what overshooting ruins.
Where it is genuinely fine
Reheating tea that has gone cold, if you accept that reheated tea is a compromise regardless of how it is warmed. Making tea where there is no kettle — an office, a hotel room, a student kitchen. A microwave tea is much better than no tea, and the difference between it and a kettle is smaller than the difference between good leaf and bad leaf. Making something robust: strong black tea with milk and sugar is a forgiving target and the unevenness matters least there. Where it is worst: delicate green and white teas, anything where you are trying to hit a specific temperature, and any tea you paid a lot for.