Among other things, it appears that Paddle
1. reserves the right to unilaterally vary the contract in any way, with immediate effect and without actively notifying the merchant
2. ensures that if anything goes wrong, any legal action against it by its merchants would have to be taken in another country and under another legal system, even if Paddle operates in the merchant's own country
3. takes extensive control of the product or service they are selling on their merchant's behalf, to the extent that they can sell it to whoever they want, sell it at whatever price they want, give demos to whoever they want, etc.
4. requires that any software it is selling on behalf of its merchants be bug-free, and that the merchants accept liability for anything bad that happens if it isn't.
It's hard to take any service seriously when it has terms like that in its standard agreement. Unless these kinds of terms are unenforceable where you are (or where Paddle is, given the above?) and you have a legal opinion telling you so that you trust enough to bet your entire business on it, it seems like you'd have to be crazy to accept them.
I've seen some very one-sided terms from other payment services before, but I take them at their word that those terms are typically required by the giants behind the systems like the card networks and banking groups. I think some of those should be also be unenforceable by law, but right now that seems to be the price of admission. I've never seen anyone else seriously suggesting the kinds of terms that Paddle is, though.
Edit: Changed to present tense, after checking Paddle's legal terms of use at the time of writing at https://paddle.com/legal/ to confirm that these criticisms are still current. The first two points above are in the preamble at the start of the document. The third is under 4.2 and 6.1. The fourth is under 13.1(ii) and 13.2(ii).