I come across lots of videos on Lithium Ion as direct replacement for NiMH batteries on Prii. Lots of local workshops in my country specializing in hybrid started offering this replacement service as well. Prices are like 20%-30% cheaper than NiMH replacement from Toyota. I am wondering if this is safe and will work properly as they did not replace the BMS. Any thoughts?
That is exactly the $64,000 question (or use whatever value you put on your Prius). There's a lot of existing content on PriusChat about that; you should probably start with something like this search. One thread that will come up in that search is this one, where someone is pretty far along in designing an actual proper BMS for a lithium replacement: https://priuschat.com/threads/libsu-redesigning-the-battery-support-unit-to-support-lithium.249158/page-3
I have done a quick search but only found discussion for this lithium ion replacement that is 6-7 years ago. As for the Libsu replacement project. I do follow mudder and knows about his endeavour. Great project. However, this new batch of Lithium ion seems to be flooding our local market and the pricing is reasonable. This means huge potential for replacement of the old NiMH. As the value of Prii are really low and lots of users are not willing to fork out the kind of money for libsu project kinda quality. I might actually order a set to test it on my Prius C. Just want to hear what the community here think.
The search I linked above is ordered by recency; you will find plenty of content less than 6–7 years old and it will be nearest the top. I don't yet know what "kind of money" an LiBSU-based project might end up being. But before I would try to be penny-wise by putting Li batteries in a car with no Li BMS, I would carefully review the content linked above.