If i were you I would change old subfloor because level difference i so high that underlayment may not work

I thought about it but there are no differences more than .5”. im not comfident if i remove the old subfloor and spend the expensive amount to replace that a new subfloor would be any more exact.

do you feel that in theory an underlayment with engineered hardwood would be ok? my first time installing this type of floor in my home. I cant imagine any problems but wanted to doublecheck

Manufacturer dependent from what I know. Check the instructions for your flooring.

Not sure what an underlayment gets you regardless of any floor finish material chosen. The underlayment will follow the sag (if it was a "cup" a certain thickness can span over it, but not a long "sag"). You can raise your joists from below if they are sagged; you can sister sagged joists from below; you can lift the sheathing level and shim on top the joists from below; you can apply a leveler on top the sheathing (e.g. SLC).

FWIW the plank should be on edge to determine gap. Or a 4'+ level.

@3onthetree but you would lift the sheathing and shim below for 1/8” to max .5” differences?? seems unnecessary to me. and i used underlayment with all my vinyl floors in other rooms, and it did exactly that, evened out slight differences. just my experience

Your 1 little picture looks like a long, low sag, so underlayment, which is fastened every 6"-9" or so to the sheathing only, would follow that sag.

I appreciate your input but actually my question above was "Can I use an underlayment under engineered?". I had similar slight sags in other rooms, and a thick underlayment evened out the floor perfeclty in my other rooms where I did vinyl. I have heard people say no underlayment under engineered or hardwood floors, but I can't for the life of me understand why that would be the case.

An engineered wood plank does not know whether it is sitting on 1, 2, or 12 layers of plywood. As long as you account for fastener length is what matters.

I'd reduce the .5" dips before trying underlayment. Our flooring guys used asphalt shingles.