I should have said that it's very limited and much more difficult to use than Ethereum's smart contracts.
I also wanted to avoid going into a deeper discussion that would involved the potential adoption of segwit, which would enable lightning networks and a bunch of other sidechains.
I think the biggest issue with Bitcoin Script is that only a subset of scripts is considered "standard" [0] and only standard transactions are propagated by the network. If you can find a miner that accepts non standard TX then it's fine and blockchain already contains some interesting transactions (like lottery [1]).
[0]: https://bitcoin.stackexchange.com/questions/21123/which-scri...
[1]: https://curiosity-driven.org/bitcoin-contracts#multilottery