Welcome to Librarium Online!
I have been reading new codex, and new rules for kroot hounds are looking really good. They have 3 attack on charge, Ini better to marine and there could be 12 hounds together with 10 kroots - really cheap unit for static tau. They could sit behind units of FW and countercharge anything that get close. They hit and wound most units on 3+ and could destroy small units just with numbers of attacks (10 kroots + 12 hounds gets 66 attack on charge)
What do you think, is this unit usable or its no save will be too big disadvantage?
If it is indeed used to countercharge, I would imagine that it would be a game turner. The no save is a problem, but with the high Initiative, hopefully everything will die in CC before they strike back. Sounds like they have finally given the hounds what they always deserved.
This would be quite useful; just make sure you are within 6-9" of the firewarrior squad in case the enemy massacres them. Or just throw photon grenades on those firewarriors.
Kroot are more or less a 1-time-use, unless they absolutely massacre. Their low or nonexistant save and low toughness means when the enemy attacks back, most things will wound and kill off a model.