Harnesses in a road car ain't too bad...
...if:
You're the only one driving it
You've got seats that fit the harnesses well (harness slots, so that you're not hunting for them lost down the sides all the time)
You've got long enough arms to reach all the controls from the driver's seat
You've got a FastTrack/EZ-Pass/Other automatic toll booth paying widget
You never park in barrier car parks
You buy harnesses with good adjusters (pull-up on the lap belts, pull down on the shoulders)
You fit the harnesses with the proper belt angles
I had TRS harnesses in the Land-Rover on Rover Vitesse seats. Not *too* bad as a daily drive. You sat "on" the car rather than "in" the car so the adjusters were easy to get to; it had no radio/heater etc that you had to reach for; I moved the handbrake within reach.
Same harnesses in a single-seater? Absolute nightmare. Impossible to get them synched down tight without two people helping either side of the car with their feet on the buckle pulling like crap on the lap belt adjusters. Much the same with the shoulder belts. Not enough elbow room in the cockpit to do it yourself, and you'd never get the buckle done up if you tried to adjust first.
I've had Scroth harnesses in a '5 with Corbeau clubmans (terrible, terrible seat - just don't) and that was workable but not great. You soon learned to keep the door open (but not too open, else you wouldn't be able to reach it) after getting in so that you had the elbow room to do the (pull up) lap straps. Wheel/stalks and gears/handbrake you could reach, but nothing else. No chance of doing the roof without undoing the belts, and if you dropped your change on the floor that game over.
***The Scroth harnesses *are* leagues ahead of anything else I've tried in the adjuster department, but that doesn't diminish the rest of the 4-pointer faff ***
Best belts ever were in an old Rover 800. Inertia-reel 3-point belts, but if you would them out all the way before putting them on it would engage the ratchet. So as they reeled in they wouldn't then come back out again, unless you undid the buckle and reeled the belt completely in. Quick to put on (click it in, pull the belt out, let it reel back in, then snych it tight) but kept you nailed in place. Mostly the tight lap strap that did it - you could slighter out from the shoulder strap to grab things in the cockpit if needs be, then slide back into the belt. Or just click it off, do what you needed to, then set it again in no time.
The lock-off jobbie you can get on ebay for inertia-reel belts are a similar way of achieving the same thing.
Some 4-point harnesses with automatic synch-down would be mega. Get it, press the lap button to synch the lap belt down, then press the shoulder button to synch the shoulder belts in, with a temporary "release" button for the shoulder belts so that you could grab things from the cockpit. Easy enough to design/manufacture, but getting those past the safety nazis is a no-go. (too small a volume to pay for the testing)
