We've upgraded a similarly large project and for us the two following mistakes caused the majority of headache:
1) Adapting libraries without regards to long term maintenance. I.e. single person project that he/she does because a current client needs it, without planning on having to take over stewardship of the library or fork it when the person eventually abandons it.
2) Adapting libraries that hook into rails internal apis, without writing this shortcut up as a debt that you might have to pay later when the private api changes.
1) Adapting libraries without regards to long term maintenance. I.e. single person project that he/she does because a current client needs it, without planning on having to take over stewardship of the library or fork it when the person eventually abandons it.
2) Adapting libraries that hook into rails internal apis, without writing this shortcut up as a debt that you might have to pay later when the private api changes.