I'll start off by saying my carpentry skills are fairly basic, but I want to learn to tackle more myself. I'm more mechanically oriented, but I generally enjoy DIY projects. I'm also a bit of a perfectionist, so doing my own work usually takes time, but I end up with a better result than when I pay people. Anyway after getting some quotes to install hard wood flooring in a section of our house I decided to attempt it myself. What most of the videos I've seen online don't prepare you for is the unlevel subfloors that I found under the carpet. It seems like quality wasn't a concern when they framed the house. I had a friend that is a custom home builder come by and help me with a few small tasks to get me started. He told me to get Mapei Self Leveler Plus and put it down in the low spots on the floor. I started this process and did several spots, but I'm really having concerns. One section has it almost the length of the room near the wall. I have another client that is also a builder and after telling him about it, he seemed really nervous about me using that under flooring that will be nailed down, rather than floating floors. His concern is it breaking away when you shoot a staple or cleat through it, but he's never done it and is curious to hear how it goes. This lead to further Google searching with some people saying they shot staples through it and it's fine, while many others are saying you can't nail through it as it'll crack it and lead to it crumbling over time. I've seen suggestions on everything from putting shingles over the low spots, using multiple layers of roofing felt in the low spots, using cedar shims, etc. I talked about this more with the guy who originally recommended it and he didn't seem concerned and seemed to think it didn't really matter if it cracked under there since it was just holding the flooring up and the staple was going into the actual subfloor and joist. Now I'm kind of at a loss on where to proceed. I just don't want to end up with a huge issue 5-10 years down the road where I'm having to redo all of this, as it's been a ton of work and I haven't even gotten to the point of laying the floor down.

So my options now are to attempt to chip this stuff away from the plywood and go with either many layers of felt, or a shingle, or similar. Or to cut the subfloor up in these spots and use a shingle or layers of felt to level it from there. Or option #3 is to leave this stuff and staple through it. What are your opinions? Am I going to have issues down the road from this stuff? It'll be a pain to remove/replace what I've put down, but I've only done one room and it'll be much easier to do it now rather than after I lay the flooring if using this stuff is a bad option.