It will ALWAYS work right, there is just a matter of performance issues on some systems. uTorrent makes many writes to HDD and because of that, avast! scans many parts written to HDD many times, creating quite severe CPU usage and HDD activity. That's why it's disabled by default. It's not because avast! has problems, it's because of how uTorrent works and avast! can't do anything about it. So they decided to disable it by default. File System Shield will still fully protect you though.