There are a lot of complaints about bugs in HS4 from many users. It really depends on what you want and what you add to your system. A lot of it is people not liking the new UI over the old. HS has been doing a better job last several months fixing HS4. I have a lot of custom scripts for my system that would need rewriting and I have no time for that right now. I would search on the forum. There are plenty of threads complaining about HS4.
If you run HS on a pi, there are limitations on the number of plugins you can run... at least with HS3 (not sure about HS4). I'm not a Linux guy, so I've stuck with windows solutions over the years. I've had low powered setups in the past, but always ran into issues down the road with needing more computing power. A couple of years ago I migrated to an Intel NUC i5 and have had no further issues. Remember, home automation is addictive. Your starting system will grow quickly. I have over 20 plugins currently. I have integration using plugins for Ambient weather station, Google calendar, Ecobee thermostats, Unifi platform , Iphone location, Sonos with whole home announcements, Caller ID with announcemets, etc etc. Scripting is where the real power of HS is, where you can do just about anything you want. I have no background in computer programming, but taught myself enough of VB.net to write scripts that do whole house announcements and control my Hunter Douglas shades, for example.
I use the homeseer Z-Nets which are just pi units running HS software. These can connect via wifi or ethernet. When I first set things up, I only had a few devices and as I grew I kept adding more devices on the same network. I eventually had nearly 200 devices on one large spread out network that had issues with delays and connectivity. Solution was to add more Z-Nets on independent networks (which took several days of down time switching things around and rebuilding). Other issue was the wifi connectivity could be spotty with disconnects, so I ended up running cat 5 (we are good at that, right?) to the Z-Nets. Zwave is now rock solid. I have lots (20+) of Aeotec motion sensors in the ceiling that will turn on lights as I walk about the house. If they don't turn on instantly, I will know. Only rarely, say once a week, is there a few second delay. As far as batteries going bad... there is a free plugin (of course) for monitoring battery life that alerts you when they run down. That being said, I have a new project where I am converting my motion sensors from battery to power using 12v power. I have done this for a dozen so far and it works far better.