There are a number of things. The first is the feature creep. It is designed to do CW, but it does it quite poorly. The first 2-3 characters are dropped when you send. Secondly, the new version has a fancy screen that at least upon release, was tremendously slow. Lastly, it took them several versions to get it harmonics down to levels that were legal in the US. Before that, tons of modifications were required. The design is solid, but the PCB layout is poor.
The other thing is the PA. The layout is very poor. The higher the frequency, the lower the output. It's great at 30 meters and below, but 20 meters and up suffers. The BITX40 suffers from this too, but it's a single band radio and at 40m it works fine. I replaced the IRF510 with an RD15HVF1 and it worked great on 20M too. Mind you, this isn't an issue with the IRF510- it's an issue with board layout. The QRP Labs 10W PA uses two IRF510's in push/pull and can even work on 10m at full output.
TL;DR: Over complicated, poor PCB layout, feature creep.
That's not to say that Farhan isn't a nice fellow and a great ham- he definitely is! They just let this one out too soon before all the bugs were worked out, and allowed compromises that shouldn't have made it into production.
Honestly, the version they have is about as refined as they are going to get with it. There have been software changes that make the display better, and it's hackable so that you can make your own VFO for it etc, but the design has some fundamental flaws. To put it in perspective, v5 and v6 are almost identical except for the display. All of the same CW, layout, output, etc problems exist on the v6 as they did on the v5.
Keep an eye out for the QRP Labx QSX. It's been in development for a long time but will probably blow everything else out of the water, even things costing 10-15x as much.
