Well, what a lot of caver-cyclists are out there it seems. I came to use SPD clips later in life than most and find them good in terms of stability and applying power when needed. Very happy with this system. I've never got knee trouble from cycling with either flats, old school clips or clipless, or mountaineering for that matter - it's running that does your joints in. I've got open-frame "old school clips" without the straps on my mountain bike but SPD on the road bike. I have flats with screws in them on the opposite side of the road bike pedals to have a choice though the downside of that is having to check if the other pedal needs flicking over when getting away on the bike.
I was really wary of having a bike attached to my feet; this felt a bit like trying skiing as a beginner. You feel so daft with long plank things attached to your legs while stood over a slippery drop. So I practiced using SPD cleats on a grass football pitch first, in case I fell off - which didn't happen, and then I suspended my bike with SRT rope off the rafters in my (big) shed so its tyres were an inch or two off the floor, and practiced engaging/disengaging SPD certain that I could not fall over. I've got Shimano shoes which are comfortable to use, though I had to buy 2 sizes bigger (in European numbers) than my normal for walking boots - the people at Shimano must have tiny feet.
So I'd say be bold and give SPD a try, but like doing anything new, put the skills in place within a controlled and safe setting away from traffic. Clipless riding skills and confidence should come quickly, unlike the input needed for skiing and SRT proficiency.