They are partnered by the third-party exchange and that's Changelly exchange, so it means they have a fixed rate regarding the fee which is you might blindly be accepted the possible high fees. Most likely an instant swap has these terrible fixed fees that could hurt your balance during swapping and I think that's good advice than choosing a single wallet for Bitcoin and convert it when you needed.
Instant exchanges definitely offer worse rates than CEXs and especially DEXs.
I have never used Changelly, or anything similar to it, but at the end of the day, how many coins would actually be transferred to your wallet when you use Changelly compared to a centralized exchange, for example? I am using a CEX as a comparison, because they are far more popular and record higher trading volumes than DEXs.
It's important to note that with Changelly you will get a worse exchange rate, but you pay no withdrawal fees.
When you use a CEX, you can set your own buy/sell orders, but you end up overpaying on withdrawal fees. Many CEXs still require 0.0005 BTC when withdrawing for example.
I would be interested to see a real comparison to know what is the final balance of your wallet when instant exchanges battle it out against CEXs.
