I expect lots of graphical errors, and since the TIA is doing the graphics and sound, probably bad sound also.
The CPU and I/O chips are simple to recreate compared to the TIA which made the system display without using a RAM buffer which led to all kinds of tricks and exploits it wasn't ever designed to do.
Even now, the Stella emulator is getting a better TIA implementation in 2017 with Version 5, and sound is to be improved a few point versions later. So simple yet so complex TIA package of timers, dividers... the TIA documentation is scattered all over and sometimes just plain wrong information, I'll be truly impressed if they get to "acceptable" using emulation or FPGA modeling.
It can't possibly compare to Stella which has had decades of coding, and Stella emulation isn't as good as real hardware.
If it is good enough to ignore its flaws, it should do just fine.
I spent the $380 for the XRGB mini Framemeister upscaler for the convenience and future-proofing, and the composite, s-video, RGB upgrade kits and PS2 native RGB or Component, and HDMI-in gives me 480p 720p to 1080p with scan lines or without for every console and Pi emulation I own. Okay the Fairchild VES is RF only, but I have a tuner box with composite out (like a VCR without the recording part) that technically I could run through the Framemeister, but as long as I have working CRT TV's...