I have bought many third party rechargable batteries from those sites over the years. Yes, slightly lower charge capacity compared to original, but no fires. And, yes, I know my sample size is small!

One ebay sourced Chinese transformer that blew its back off (!) letting our the magic smoke and tripping the RCB was enough for me.

These days I'm older and wiser.

I also had similar good experiences buying batteries on Aliexpress. The issue with those typically isn't intrinsic quality as the batteries are most of the time good but lack of quality control. Bad batteries will reach the market and this is specially dangerous with packs with many cells like e-bikes packs.