Hey Thomas,
thank you for your response, I appreciate it!.
To be honest, the main thread was the one with Builder Lag, that I already mentioned before. But soon after I posted, we got a response there - so that is good 
(Another thread about lag)
The other one, that got mentioned many times in Discord is simultaneous editing, for example this one. - and I know its still a bug, because it happened to me not long ago
Those two are the ones that get mentioned the most. At least those two I know from top of my mind.
And again, thank you for your response Thomas!
EDIT: Ah, I see Adrien beat me for just few minutes.