Upsides:

Smaller and often more local than regional, national and international banks.

Less onerous requirements for loans.

More favourable terms for loans, deposits and passive investments.

Downsides:

Access to fewer ATMs, teller locations, but many request fee free ATM access with a major bank so you can access your money easier.

Usually worse or the same kind of opening hours as banks.

Banks often have discount offers on insurance and various other partnerships with their leftover money, credit unions it happens less often.

Various means of communication/correspondence. Some are only online, so only send notices by letter mail, etc.

Not being able to do some things as easily, i.e. send money orders, domestic and international transfers, write cheques or cash them, have a safety deposit box. Most people don't use or need these things as much. All of these points vary by each credit union though.