> It's strange that the author encountered difficulties in soldering XT90 connectors; I actually never soldered more than a few of them but never had any problems; it is possible that either they were of dubious quality (far east clones with possibly sub par copper contacts?) or they weren't clean enough.

Or simply use too small an iron for the job. XT90 isn't the problem here, more likely it is the fact that the wire drains heat so fast that you don't get to a nice temperature.