What it does well
A cafetière gives loose leaf plenty of room to move and open, which is more than most infuser baskets manage and is the main thing whole leaf needs. It is glass or steel, so it is non-reactive and easy to see into. It has a lid, which retains aroma and heat. And you already have one. For large-leaf teas that need space — many oolongs, whole-leaf black teas, white teas with big open leaves — it is better than a mug infuser and close to as good as a plain pot with a strainer.
Problem one: the tea keeps brewing
Plunging separates most of the leaf from most of the liquid and it does not remove the leaf. The compressed bed at the bottom stays in contact with the tea, so extraction continues, and by the second cup the pot is over-brewed. This is the same issue cafetière coffee has and it matters more for tea because tea's late-extraction fractions are so unpleasant. Three fixes. DECANT the whole brew into another vessel as soon as it is ready — the simplest and most reliable. Or brew ONE CUP'S WORTH at a time. Or lift the plunger clear and use it only as a strainer as you pour, which works if you pour promptly.
Problem two: the mesh lets fines through
A cafetière filter is a coffee filter, sized for coarse grounds. Small tea particles pass straight through, so a broken-leaf or bag-grade tea gives you a cup with sediment in it and a gritty last mouthful. Use whole-leaf tea and the problem largely disappears. Use fannings and it will not, whatever you do with the plunger. A related nuisance: fine particles clog the mesh, and a cafetière used for both tea and coffee holds onto oils and aromas from the coffee. If you use one for both, the tea will taste faintly of the coffee, and no amount of rinsing entirely fixes it. A dedicated pot is the honest answer.
How to use one properly
Pre-heat the pot with hot water and discard it — a cold glass cylinder takes a surprising amount of heat out of the brew. Use the ratio you would use for a teapot, around 3 g per 200 ml. Pour water at the temperature the tea wants; a cafetière does not change that. Leave the plunger UP while it steeps, so the leaf can circulate. Press only at the end, and press slowly — pressing hard forces liquid through the compressed leaf and extracts the harsh fractions directly. Then decant. Everything else is ordinary tea brewing.
Where it is the wrong choice
For finely broken or bag-grade tea, because of the mesh. For gongfu-style brewing, because the volume is wrong and you cannot pour off fast enough. For matcha, which is not brewed at all. And for very delicate low-temperature teas like gyokuro, where the thermal mass of a large glass cylinder works against you. For most ordinary loose-leaf tea it is a perfectly reasonable pot, and telling somebody to buy a different vessel when this one is already in the cupboard is advice that mostly serves the person giving it.