…from the author website:

Since the end of 2015, it became clear that the “good old” 1541 Ultimate-II was at the end of its life cycle. Xilinx, the manufacturer of the most important chip of the cartridge, had started to increase the prices of the chip. Also the memory chip started to be harder to get and at higher cost. When the news came that the company that made the black plastic cases for the 1541 Ultimate-II had lost the injection mold tool, it became evident that the 1541 Ultimate-II would be no more.

I have given it some thought. What would be the best way to continue with this project? When the feelings of being upset about the lost injection tool had subsided, I realized that this may be the perfect moment for an upgrade. This is when set backs change into opportunity.

Since then, a lot of work, and I really mean A LOT OF WORK has been done to create a successor: the “Ultimate-II+”.  Why not “III”, you may ask or think. Well, the foremost reason is the software. The install base of the “1541 Ultimate-II” is rather large (>3000 units), and I do not want to let my customers down that have just purchased a unit. This means that in case of new hardware, the same software should run on both the new board, as well as on the older board. So, the new board will offer similar functionality. You could see it just as an upgrade, not an entirely new product.

Although the “Ultimate-II+” is similar, there are also differences:

• No more MicroSD!  (… that needs to be pushed in with something sharp, could fall into the case, … etc)
• More USB ports (… one to compensate for the lost SD, and just one bonus: 3 in total)
• IEC connector moved (… further back to facilitate use on some C128 models)
• Integrated Ethernet (… no more need for an external USB2LAN adapter, and a smaller step to CS8900A compatibility)
• Dual Flash (… no more worry that a software bug or upgrade failure leads to a bricked device)
• Integrated Speaker (… who connects a speaker set to the audio jack for hearing drive sounds??)
• External power supply connector with auto-switch over circuit (… for stand alone mode, and to lower the demand on the power supply of the C64)
• Improved Audio (… audio codec, 24 bit/96 kHz*, for sampled sounds and SID emulation)
• Improved Tape connector (… no more custom board and able that snaps in on the bottom, but standard USB 3.0 cable)
• Improved power supply circuits (… for some reason they seemed to break down relatively often)
• Slightly bigger and faster FPGA, as to control the new features

source: 1541ultimate.net