I have tested it with many video formats and some videos in the slideshow hangs at the end of the video and the app "crashes". The video displays only the last video frame and then stopped.
At first glance, it looks like that the problem are only by AVHCD videos at least by me. MP4 Files from iPhone or similar are working well.
I play the slideshow from folders that are hosting on my Synology NAS over SMB on 1Gb LAN.
I tested also with a Raspi 4 and Kodi and OpenElec and there is working without any problems.
But I would prefer to use my ATV4k and MRMC and not my Rasp.
FYI I using at the moment the MRMC Lite app, but I would like to buy the app of course if the slideshow is working well
Edit:
For the test, I've create the videos and picture in alternating order:
01.mp4
02.jpg
03.MTS
04.jpg
05.avi
06.jpg.............