Pros
Large bank comes with security but also with it's own share of politics. Also, most major departments have a US vs. Canada is split within their organization hierarchy at various levels. And the US heads almost always report up to a Canadian. Other than job security, little pros for someone looking to grow their career beyond just the limited scope of US teams.
Cons
BMO is truly a Canadian bank and as such wages are significantly lower than even peer US banks in LCOL areas. BMO's salary ranges for CA are some of the lowest I have ever seen. US Banks based in IL, TX have higher salaries for the same job level than what BMO does in CA. BMO may have recently entered west coast markets but it's going to have quite a bit of learning to do if they want to use the location to their advantage to attract top talent, especially on the technology side. With the pay bands they have there is no way they can hire anyone other than remote workers living in very LCOL areas. Even an entry level engineer in the bay area probably earns more than what BMO is willing to pay their managers.