Speed and temp
I've seen the old Luhr Jensen speed device and if there were not such drastic under surface currents on Lake MI it would suffice. There were days this summer when I had to go more than 1mph faster one direction vs the other to keep the speed at the ball the same.
I'm a month late but here's my take. They all work. Back in the 70's we used the old tried and true Luhr Jensen manual indicator based on angle of ball put into the water. We caught fish. This was before GPS's or speed and temp at the ball were available.
A Sub - troll. Fished on a boat that had one but never owned one so I won't comment.
The Depth Raider did have problems with cable coatings. I think the thicker coating has solved the problem. Mine went with the last boat.
The first Fish Hawk's had some issues also. The new X4 is the cats arise.
I'm not sponsored by anyone so my answer is unbiased. You're going to see 1.9, 2.2, 2.0 or as high as 2.7 on any speed at the ball given the currents bounce up and down. More so this year then in the past.
I did have a problem with loss of signal >120' down that was due to noisy alternator on my Yamaha kicker. It interfered with the signal and reduced its strength. I got a noise suppressor from Curt Kell that cleared it up.
The Depth Raider has something in their software that dampens the speed readings so they don't bounce around. The drove me nuts on my first Fish Hawk but sounds like they dealt with that issue.
I chose FishHawk for one reason: No coated cable. I have seen nothing but problems with the coated cable from Depth Raider, and it's a PITA to crank in, if you have manual riggers. Other than that, I think the units are fairly comparable. I can't comment on the SubTroll, as I have never been on a boat with one....