Well all new coins are proof of concept before they gain adoption, so this isn't really an argument why a technology is bad.

Skycoin's consensus algorithm is among the few that are peer-reviewed next to Cardano, so i think that's as much credibility as you can get as a new coin.

Iota's coordinator is expected to be decentralised and then removed next year already.

In order to gain control of the network, you would need to gain 34% of the hash power, omnipresence in the network and omnipotence. I'm not sure what the cost of this is, but there is currently no more secure decentralized network design that I know of.

Keep in mind that 45% of Bitcoin's hash power is currently controlled by one company, Bitmain, and they are investors in the 3rd biggest mining pool, totalling 75% hashing power of the entire Bitcoin blockchain.