So the 29W adapter looks exactly like the 30W adapter, and if you make the perfectly reasonable but totally wrong assumption that the stated maximum wattage is all that matters, it sounds like it’s about 97% as powerful (29 ÷ 30), but what really matters when an adapter is negotiating with a device are the various voltage/amp configurations that the charger can supply as output, and the device can accept as input.
For Tim Cook, who so often measures hardware success in units of “customer sat”, this charging situation has gone completely out of hand. Apple needs to do two things. They either make two chargers, one for iOS devices and another for MacOS devices each with a USB-C port as the termination on the back OR they need to very clearly label in some sort of embossed etching on every charger they make the wattage.
I’ve been lucky because, for the last 10 years or so, I keep my chargers, headphones and more in the boxes the devices came in. At my office, I have a wall of iPhone boxes for privacy and I just have everything sitting on wireless docks. Because I’m always charging (bedside, car, office), I’m never less than 90% charged so a 5 or 12 watt Qi charge, doesn’t matter to me, I’m always charging but if you’re on the go, speed matters and the HomePod mini, Magic Keyboard and iPhone 12 Pro has shown that Apple really need to get chargers under control. My customer sat would be decimated if my phone was dead on a business trip and I plug in to the AC adapter at a bar for 20 minutes and only get 5% additional battery on my iPad or iPhone.