Sadly this is probably correct. Larian went on to say that they added it to spice up the endgame where you have less level ups so so if you want a healthy brain then you probably get jack. It is stupid to me that Larian fixes this problem only for one group of players and doesn't seem willing to fix it for others. And I don't mean being op if you were using it, but just content for people that want to play good, because so far it looks like playing evil will get you more interesting gameplay while playing good gives you nothing except for pat on the back at the end. That's like in WOTR there were only Lich, Demon or Swarm and there was no Angel, Aeon or Azata. There needs to be balance in this case and Larian so far seems to favor one side and ignore the other one. That is not good way of showing "temptation".
I don't know about that. To me, it sounded as if you get a lot of power with using tadpole powers, but will loose your companions. As team healthy brain you have your companions to help you. The only companion, that I can see staying, if you start experimenting with the tadpoles, is Astarion. The others made it very clear, how their views are, Lae'zel will probably even attack you outright. Halsin and Jaheira might too, given their opinion of the tadpoles.
Edit: I'm referring to Swens interview, that was posted here a while ago, where he said, that with using the tadpoles, you will be become very powerful, but lonely.