This being 40k, not as much of an "i win" button as many would imply. A major strategic boost, at least after some more work and exploration of the webway is done (navigating it is generally considered a major issue even once someone gets into it) but probably not a full replacement for warp travel, especially for first few millenia. "Borrowing" Eldar enterances/exits to the webway is not feasible apparently, if it was the Imperium would be doing that since ages. That's why the webway project was such a big deal. The Emperor has figured out a way to build own portals and "enterance tunnels" to the proper webway, with AdMech assistance to do the building. The problem is that even they could not figure out how to do it up to the proper webway tunnels. As we know, they are built in the warp...
That's why the human built tunnels required wards and the psychic protection of at Emperor\Magnus grade psyker sitting on the Golden Throne to power them (which Magnus broke with his psychic message). So it seems like quite a finicky thing, with catastrophic consequences in cases of failure. Suddenly doesn't seem that much better than warp travel. Now the AdMech is in worse shape in many ways than in M30, Emperor can't directly take part in the research on the project, and there are questions about his psychic condition - after all, the project was still being worked on, and it was meant to be powered by living Emperor, not a kind of sort of artificially sustained one. It raises further questions - how many human built connection tunnels can he reliably shield? Can Admech still build new ones properly, and at what rate? What happens if knowing of it and the strategic advantage it gives the Imperium, all the neighbourhood chaos sorcerors start trying to intentionally pull off what Magnus did and cause warp breaches in the connecting tunnels, not necessarily on the one connecting Terra to the webway, but also more obscure and less well protected places?
Even if those problems are handled reasonably well, we know that the webway itself is not exactly in top shape, and is not connecting all places in the universe anyway. Many sections of the webway are blocked off or in unusable state for various reasons...