This may be true if you consider maintenance as "fix if something is broken". If you follow the inspection guide in the manual the routine swapping of parts like stem and fork parts in relatively short intervals make it definitively far more expensive. Plus you loose your warranty on the frame if you do not get a stamp in your book for a yearly inspection with a birdy dealer.
The required parts-swapping may seem to be a bit on the paranoid side of life (and therefor up to everybody's own judgement) but I've read a couple of incidents with those (and other) parts breaking even on newer birdies w/o mistreatment, therefor it doesn't seem to be totally irrational.

Regular parts swapping plus yearly inspection make it an expensive bike in terms of high running cost (plus a high price to get one at first). If you are willing to take the risk with parts breaking plus you don't care for the warranty on the frame (or you do not have any as you bought second-hand) you can run it cheaper.

Well: every part related to frame, stem and fork is non-standard (aside of the suspension). The front hub is uncommon at least as is the seat post. The racks are non-standard. The rims and spokes are uncommon. And so on and so on. I'd assume there are more non-standard-parts on a Birdy than standard ones. Standard is basically the rear hub, everything regarding shifting and breaking, the saddle and the bar. That should be pretty much it - most of the rest is more or less non-standard or at least uncommon. There's nothing wrong about that in my eyes - that's the price you have to pay for a special bike.