A year later and I have just rebuilt my Unifi controller on a DietPi Buster based VM using this script and also created a LE SSL certificate and used his LE script to install it (you need to do some weird keys thing).
Really easy 
[edit]
I have noticed in the past that the memory ran away - it looked like a memory leak. Actually it turns out that, by default, the java app is allocated 1GB of memory. Not so good on most SBCs. This is easily solved by allocating less memory…
nano /var/lib/unifi/system.properties
And add these 2 lines (adjust sizes to suit).
unifi.xms=256
unifi.xmx=512
This is now super stable!