Synology has shady business practices more recently and outdated tech stack since forever but I haven't heard anything particularly bad about reliability. For a NAS the safety of the data is the highest priority. Anything that endangers the data isn't just a drawback, or a "con", it's an instant elimination. Right now I wouldn't trust Ubiquiti NAS to store my recycle bin, I need to see a long track record of reliability and a commitment to quality.