People always like to complain, whether it is justified or not
No matter what airline you choose, you have 3 options:
1. Pay up seat selection in advance and get the seats you want
2. OLCI the minute it opens, and hope you will be assigned seats together, if not pay the fee then (if still applicable)
3. OLCI the last minute and complain you are not seated together anymore... (or don't OLCI at all and complain you are charged for airport check-in)

I've seen plenty of inexperienced passengers being seated apart on KLM flights, including a couple with their lap infant seated in B + D of the same row, this would not have happened if they checked-in online, even 4-5 hours in advance.