Yes, things also do go right on KLM. The question is how to increase your chances of them going right.

I have never had an unpleasant surprise in seating on CO or NW; what I reserved with ease weeks or months ahead on their quality websites was either honored or upgraded to a higher class. Reserving a non-bizclass seat on KLM however is usually not possible and often arranged strictly for the operational ease of KLM itself not for the convenience or benefit of the paying customer.

There is a difference in philosophy. KLM sees itself as in charge of the seating whereas other companies give the consumer the choice. If you are able to score attractive seats through nwa.com or in OLCI you risk these being unilaterally taken from you and given to friends or staff of KLM or to passengers who make a request at physical check in. These are the facts.

Unfortunately, there is no way to really conveniently guarantee an attractive economy class seat ahead of time on KLM.

OLCI should open up 24 hours before the first flight of your itinerary.
If this is way ahead of your KLM international flight, you may have a very good choice of seats but risk that KLM will do a mass re-assignment of seats before the operational prep window for that flight. If it is just before the flight, you may have a poorer selection, but a better chance of keeping your seat. If you choose a bulkhead or "first row of cabin" seat your chances of losing it may be increased if there are many families with babies or small children on that particular flight. If business class is full and you have a forward cabin aisle seat or a green seatguru seat and there are family of crew or non-rev passengers flying, your chances of losing your seat are also increased.