A lot of it depends on what you want your PC to be able to do. If all you want in your PC is to be able to browse the web and edit some documents, then the Librem 5 might be good enough.
One of the big questions is how much RAM the Librem 5 will have. Purism has committed to a minimum of 3 GB of RAM, which is certainly enough to run the Linux/Wayland/GTK+/Phosh software stack on a phone, but 3 GB won’t allow you to do much multitasking if using the Librem 5 as a PC. If you like to open 20 tabs in your web browser, then you will be frustrated with 3 GB of RAM. If Purism decides to give us 4 or 6 GB, then the Librem 5 will be much better as a PC.
The i.MX 8M Quad doesn’t have a powerful CPU (1.5GHz 4xCortex-A53). The Cortex-A53 doesn’t support out-of-order execution like the Cortex-A7X series and it only has a 1MB L2 shared cache. It isn’t really designed to be a CPU for a PC. It is fine for light tasks which is what most people do with their PCs, but you definitely wouldn’t want to do anything that is calculation intensive like edit a big spreadsheet with the Librem 5.
The Vivante GC7000Lite GPU on the i.MX 8M has decent AnTuTu UX and 3D scores, so you can play 2D and 3D games at low resolutions (see Quake II on the Librem 5 Dev Kit), but we don’t know what kind of cooling the Librem 5 will have. If the Librem 5 uses a standard graphite sheet like most phones for its passive cooling, then you are probably going to get throttling. If it adds more expensive cooling like heat pipes or a copper sheet, then you might not get the throttling.
Another issue is that the Etnaviv driver only supports OpenGL ES 2.0, so you will have to install the proprietary Linux driver if you want to play many of the recent games that use OpenGL ES 3.0/3.1 or Vulkan. I wouldn’t count on much gaming on the Librem 5.
A big question is whether we will be able to use FOSS drivers for the Hantro G1/G2 video decoders. The Linux kernel recently added support for MPEG-2 and H.264 in the Hantro G1, but there is still no support for MPEG-4, VP9 and H.265. I have seen commentary that we should get FOSS drivers for the other codecs, but for now we have to use software decoders, which means that you might get overheating and throttling issues when watching a long video, and you will certainly get very poor battery life. I assume that people who want hardware video decoding will install the proprietary Linux drivers for the i.MX 8M.
Another question is what kind of external screen the Librem 5 will support. We know that you can only have one external monitor because HDMI alt mode and DisplayPort alt mode over USB 3 don’t support dual display mode and the i.MX 8M doesn’t support it either. The i.MX 8M supports 4K at 60fps with both HDMI and DisplayPort, but the HDMI is only available using a binary blob in the i.MX 8M. Unless Purism adds a separate chip to convert from a MIPI-DSI display to HDMI, we are only going to have DisplayPort. (Purism will have to add a USB-C host/Power Delivery chip that can convert from DisplayPort to DisplayPort alt mode, since that is also not supported by the i.MX 8M.) Just because the i.MX 8M supports 4K, it doesn’t mean that the Librem 5 will support it, but even if it is supported, you are probably going to want to install the proprietary drivers if you want 4K video and forget any high-resolution gaming.
The final question is what Wi-Fi/Bluetooth chip will Purism use, which will be needed for the Bluetooth keyboard and mouse. Purism was working with Redpine Signal to produce an 802.11n chip that can run on FOSS drivers, but recent announcements have not included any mention of Redpine Signal, so I suspect that either the project was abandoned or it is not yet ready. Purism probably found another Wi-Fi/Bluetooth chip that it can run over SDIO using a FOSS driver, but we have no idea what it will be and how well the Bluetooth will work.
I don’t know what kind of software issues are involved with making convergence work, but given all the major challenges that Purism is facing, I wouldn’t bet on convergence working well from day 1 when the Librem 5 is released. Frankly, if you need convergence, you are probably better off waiting.
Purism has shown with the Librem 13/15 that it will keep improving the software over time, so I wouldn’t give up on convergence if it doesn’t work very well on day 1. However, I don’t see a clear path for Purism to make convergence work well on the hardware side. If Purism moves to the i.MX 8M mini in future versions of the Librem 5, it will have much better energy efficiency, but it will have even less ability to do convergence. The i.MX 8 QuadMax has the horsepower to run a PC, but it would never work as an mobile SoC for a phone.
It looks very unlikely that NXP will decide to make a powerful mobile i.MX 8-series SoC, and the i.MX 9 is probably 4 years in the future. The best prospect for convergence is the Rockchip RK3588 (4x Cortex-A76, 4x Cortex-A55, Mali G52 GPU, 8nm FinFET) which is due in Q1 2020, but the free Lima GPU driver will have to improve a lot, and we have to pray that Rockchip will release info about the chip at http://opensource.rock-chips.com like it did for its previous chips. Hopefully, Google will decide to make a Chromebook reference design based on the RK3588 like it did with the RK3288, so we get Libreboot support for it.