Wow, not sure how to interpret your experience that a RPi wasn't powerful enough to manage watering a few plants. I can only suspect the overall software setup is massively bloated.
The important thing is that it's pretty much always easy to make an upgrade thanks to the good design of the backup system. Don't forget to set up backups in either case, it's a sin to not use such a complete system :)
…having said that, as the other poster alludes to, it’s peak requirements that are problematic. If your device can handle them, it’s not a massive power suck because idle requirmements are low.