Pros
Absolutely none unless you can make it to the higher salary bands to start receiving free benefits that lower bands have to pay for like private healthcare insurance. S3 upwards can also coast nicely without too much work or effort from what I noticed.
Cons
If you're S1 or S2, Santander WILL set unrealistic expectations for performance and deliverables, overwork you, strip away any and all perks of the role that might make your life easier, manage you out if you require any reasonable accommodations due to health issues and generally treat you like a robot that exists only to be milked by the company. Progression from S1 and S2 is almost non-existent, promotions seem to be few and far between while internal recruitment feels like you may as well be applying as an external candidate; management will hinder you if anything. Pay seems to be lower than market rate across the board, from branch/call centre to senior leadership. Words fail to describe how hollow and soulless my time there managed to make me feel. Company needs to completely change its approach to lower level staff if they want to avoid the sky-high churn they have now. As it is, I would not recommend it to my worst enemy.