The Tunze has mixed reviews because the vast majority of people don't understand that it's not going to produce a dry, high volume skimmate and conclude that it's not working correctly. Then they conclude that it's not adjustable, but it's not meant to be as adjustable as many designs.
If left to function as it does it will keep your tank clean while needing much less attention. Which it will do for years, hence the cost because it's built very well. Long after you've replaced the DC pump on the Bubble Magus the Tunze will be running the original. Long after the BM has cracked while you're cleaning it the Tunze will still be running without issue. You can also inject ozone into the Tunze which will make the BM brittle and cause it to break even sooner.
There are many approaches to skimmer design and many levels of quality. You do get what you pay for and if the BM fits your budget then it's the right thing to buy. It'll be fine for a couple of years.
DC motors are a new trend in the hobby. They're quiet and the ability to adjust them is very useful. However the Chinese can make anything to a price point and the life of that pump remains to be seen. £45 is the entry cost for the BM. It may not be the total cost.
Sitting at my feet is a skimmer that a few years ago was the king of the hobby. I ran it for about 3 months and it's like new in the box. I paid close to a thousand dollars for it. But while the design looked really cool and it produced a huge volume of air it really didn't skim that well. The company failed and it's worth 20 cents on the dollar of what I paid for it.