Re: third generation
IOTA is currently centralised, and a bit flaky. The flakiness is partly due to being designed for machine-to-machine transfers, and dumb humans don't follow the rules as well as machines do. Might be fixed by better wallet software. The centralisation they say will be removed when usage increases sufficiently. An alternative is RaiBlocks, which is working now, but not supported by many exchanges.
Neither has proven itself as well as Bitcoin in terms of security, and both have the usual issues of being deflationary and volatile. They are also proof of stake, so don't use huge amounts of electricity like Bitcoin does.