I like the idea of the briar queen, and I do not think she is bad, but I do think she has a problem serving a specific purpose in an army hard pressed for hero spots, where those heroes need to have specific synergies.
The most direct comparison might be the guardian of souls, who is 10 pts cheaper than her and her thorns. They are both "lvl 1" wizards with about the same statline.
The guardian is really powerful at bringing models back on the table and the +1 to wound aura is immensely useful and powerful and is a key factor in turning chainrasps into truly dangerous hordes.
The briar queen has no such buffs, she is weaker in melee (not overly important) but has 10" missile attack that might do a wound or two. She has her 6 chainrasps, which is not bad to have and can provide look out sir on their own, but just being 6 will not really achieve anything major, other than serve as a bit of bubble wrap.
I just see other heroes as far more important, either due to providing command abilities or critical support, as I want a guardian to buff my hordes, and especially if I bring bladegheists I wont a spirit torment, which is still nice otherwise. I also want a commander of sorts, either a knight or vampire. This is where the problem lies for me, as I don't see how my lists or gameplan is improved by replacing any of the other heroes, and when I got my bases covered by those heroes I would probably rather want 10 more reapers or 20 chainrasps than another supporty caster.
Tl;DR: I don't find her bad, but believe there are better options for your hero slots with more synergy with the overall army.