I've been working on a big (in Rails terms) team for a few months now and we've come to some conclusions concerning Migrations. I'll start with the bad and in part II I'll talk about the decisions we made and why.
Migrations are great, but they do come at a cost. When working with a large team (my current team size is 14 and growing) migration conflicts happen. This can be mitigated with communication, but migrations can definitely be a bottleneck. Also, the process of creating a migration can be painful on a large team. Before creating a migration you should always update from source control to ensure you get all the migrations checked in by your teammates. Then, the best case scenario is when you can create a migration that doesn't change anything and immediately check it in. Checking in a new migration immediately helps ensure you don't block other teammates from creating migrations; however, it's not always as simple as adding a new table. Migrations that alter the database structure often break several tests. Obviously, you can't check those migrations in until you fix all the breaking tests, which can take time. During this time, database changes are blocked for the entire team.
It can also be troublesome to find specific changes to the database within multiple migration files. Finding which migration adds a specific column can take a fair amount of time when you are working with over 50 migration files. Naming conventions can mitigate this issue some; however, naming conventions generally require that only one action occur per file. For example, the 023_create_customers_table.rb file can only create the customer table and cannot alter the purchases table to add the customer_id column. This type of naming convention helps on searching for specific changes to the database; however, it also results in a large number of migration files.