All I can say is that I’m starting on my third year this year with my M-5 with no problems or issues. Marcum is closing out their 2019 SLA units for the 2020 Lithium models. I like the dual degree ice ducer, and the 3/4” target separation. Functions between 0’ to 160’, and with the digital mechanics, it runs smoothly and quietly.
You will probably get a bunch of comments on competitors’ products, but ultimately, which unit you purchase, learn to operate it correctly, and you should be happy with most any brand of flasher.