Here is a single drawback inherited by almost any cryptocurrency from original Bitcoin algorithm (related to the term blockchain): as transaction volume grows the calculation complexity increases as well. The more popular coin becomes the less number of transactions per second is possible. Bitcoin grew so much that only 8 transactions per second available for anywhere in whole world. Think about it - this cryptocurrency is useless now for use as an actual currency in online stores for wide use. The delays are ridiculious if you would like to send bitcoin anywhere. This situation will even become worse overtime.
IOTA is different from this prospective, the number of transactions per second is unlimited. As long as other factors are covered (such as security holes, trust by users to dev team, bug fixes with the second transaction erasing first unconfirmed transaction, etc) this coin has a bright future. But there is also possibility that developers wont improve it and some other better coin with frequent transaction scalability comes out and IOTA will loose momentum as the result.
I worry about Ether most. It seems they have not found a viable scaling solution yet -
https://www.coindesk.com/vitalik-buterin-announces-new-subsidies-for-ethereum-scaling-research/. And in the mean time, there are so many competitors which soon will come out and everyone claim it is better than Ether such as ada, eos, neo, qtum, tezos, boscoin, rsk, and the revolutionary tau chain.