There are pros and cons. Benefits may include more coordinated planning, service efficiencies if they're spread across a broader area, the perception of having more "clout"... but that can all happen without amalgamating too.
I think the major downside is political. Here in Hamilton, the councillors of the former "suburban" municipalities won't always play ball nicely with those of the original city (especially the older lower city), and sometimes that happens vice versa. It's held the city back in a number of ways. And I think the loss of local town councils in the former suburbs was a negative too -- residents lost representation.
The change was a bit different for Hamilton since there was a regional government for Hamilton-Wentworth and not a county government like Essex. So some functions were always done across all the constituent municipalities (like the Official Plans, transit, policing) and those local councils were able to weigh in; when the region was amalgamated those things carried on much as before... though there had to be an effort to coordinate things like zoning and other policies.
Today, more than 20 years (
20!!! ) after amalgamation, some of the same political butthurts for opponents of the forced marriage are still an issue and city councillors tend to operate as if their wards are their own little kingdoms. It can be very frustrating when debating something like a transit proposal, which may only directly benefit a portion of the city but would have broader benefits to the whole.