For the past years a typical PTS cycle looks like this:
Week 0: Patch X.X.0 is released and the discussions about the biggest imbalances begin
Week 1: Patch X.X.1 is released often without any combat balances, discussions about the biggest imbalances keep going with no one knowing what will happen
Week 2: Patch X.X.2 is released and includes further balance changes and maybe fixes for the issues implemented in X.X.0 but might also bring in new imbalances. New discussions about imbalances emerge or are continued
Week 3: Patch X.X.3 is released often again without any balance changes. While discussions about imbalances continue.
Week 4: Patch X.X.4 is released with the last changes to the balance most of the time. If big imbalances aren't touched here there's a good chance for the coming to the live server and staying for 3 more months till the next patch is released.
Week 5: Patch X.X.5 is released to the live server and often equals all changes that have occurred till X.X.4 without further balances.
That means we're often looking at only 2 incremental patches that can fix potential imbalances.
Granted this pts cycle the combat team changed the things each pts patch however the main issue of dots hasn't been touched.
Now let's swap over to the next big issue and that's how things can be tested on the PTS.
The overall number of players on the pts is smaller than on live this means especially for PvP the only way of testing are duels which are only a small part of PvP and don't represent OW combat as builds, CP allocation, useage of LoS and number of people included differ and make it impossible to get somewhat precise predictions how the changes will affect PvP in all aspects.
In general people can point out the biggest imbalances but it's impossible to give precise feedback as the people your fighting on pts differ from the ones you've been fighting for years in OW.
On live you can match every player you meet to a skill level and if you meet them after the patch you can give much better feedback how the changes affect different players in OW in their performance.
A good way to tackle this is to extend the PTS cycle by 2 weeks after it was put on the live server. This gives everyone the option to collect much better feedback and the ability to better classify the changes (especially for players on EU).
This also helps to tackle the lack of patches to deal with the imbalances to create a much better experience for everyone.