I'm starting too see more and more mainstream manufacturers use new Mediatek SOCs in their phones, I always heard about them being inefficient, did those got better nowadays?
I don't have much comparison but the last 2 mid-range phones I bought used them and I can't complain. I think they have the benefit of being cheap and also easy to use, putting lineage on my phone is pretty easy now compared to how it was with Qualcomm based Samsung
Custom ROMs are limited due to closed nature. Make sure whatever you want to load on it explicitly supports the chip before you buy, of that's your thing.