You work with internal frameworks which are badly documented and bring questionable value, just check the 'open source' version of Cougar on github (the documentation is a joke).
I didn't like the atmosphere in the office, I felt a lot of power struggle between the managers, constant team reassignment, lack of team spirit.
The security concerns can put undesirable pressure on the implementation decisions.
Low autonomy as a developer (very difficult to introduce new technologies/practices different then the 'Betfair way') . Basically a top-down culture where architects and managers decide everything.
Lack of connection with the business for a developer - you just get a bunch of requirements with no explanation regarding how they bring value to the business or what are the long term plans