Just took the plunge and upgraded from 16.04 to 20.04 and has a bunch of issues getting my NAS drive to mount at all (fixed by changing the sex from ntml to ntmlssp) and I can access it now but only have read permissions.
It's a synology NAS and the user I'm logging in as has read/write permissions to the share which were working previously in 16.04 and have tested on a windows machine and mac.
I'm fairly basic with my linux knowledge so let me know if you need anything else. Current fstab is
//192.168.1.228/Media /media/windowsshare cifs username=myusername,password=mypassword,vers=3.0,iocharset=utf8,sec=ntmlssp 0 0
As previously stated this was working in 16.04 and since then the only change I made was to change the sec from ntml to ntmlssp.