I've used it a couple times. Apart from one or two instances the transactions have gone through within an hour of me sending the coins to be exchanged. Overall, it should be trustworthy for small amounts under 0.1 BTC. Shouldn't be too big of a problem but keep in mind you're going to be charged a fee + transaction fees + a worse rate than market. So the estimated coins will probably be inaccurate.

People that have problems usually deal with large amounts, in which case changelly seems to hold the funds for quite a while.

If you have large amounts of cryptos to trade, you should never use instant exchangers. Use an actual exchange instead. You'll get better spreads, be charged less fees, and you don't need to deal with less trustworthy counterparties.