They keep jumping between all kinds of OAUTH domains and different hostnames for authentication, any kind of privacy cookie blocker for external websites or ....... will in 70% of the cases break the site, log you out again or just not load anything. It's an absolute mess on cross-site cookies and information being exchanged. It trips dozens of alarm bells on these tools. Using a few common privacy tools (uBlock origin, privacybadger and Firefox site containers) will make the KLM and Airfrance sites completely unusable.
Changing a flight online will more often not work either (no flights available or not possible in your booking class).
Least to speak of the 'new' booking tool that KLM now uses online, the old one was a "Everything visible in 1 overview, including full ticket cost", now you keep clicking back and forth to even get pricing for a ticket and switching between screens and huge buttons and expanding cells like it's an iPad app from 2005. I guess it needs to "look bling" even though usability has taken a massive hot.