The main chain / child chain architecture as such does not make Ardor more "centralized". The whole network is still maintained by pure Proof of Stake. There are people who sustain that PoS is a consensus method that leads to centralization, but I don't share this belief.

What is partly centralized - and is a major point I criticise and I hope still [naively] it could change some day - is the development model of Nxt/Ardor. The developers work mostly "behind closed doors". They share the source code when they release new versions, but now the license is not FSF/OSI-compliant anymore, because they're too afraid of clones.