Is the initial instability of the WB while you are waiting to fill the tanks or is there something else to be aware of?
Water-ballasted boats generally exhibit adequate ultimate
stability -- how far they will lean before going all the way over. But the truth is, water just isn't that heavy (1/11th the density of lead). It IS bulky, so you can't concentrate its mass at the bottom of a lever arm. The CofG is higher. Upshot is many water-ballasted designs may feel initially tender compared to lead-ballasted sailboats, as it takes longer for the righting moment to grab hold. You get used to the angle, but such boats may always suffer from the leeway, weather helm, depowered sails, and unsettled feel that accompany excessive heeling.
And if you forget to fill the water ballast, these boats are indeed dangerous. But the same can be said for lead-ballasted boats that forget to lower their keels.
Glad you muscled your 18' wing keel out of the mud. Ain't so easy with a 2-ton, 26 footer.
That, I suppose, is the point of most comments in this thread: everything is harder, heavier, fussier, and more expensive with a larger boat. If your goal is trailering all over, launching anywhere, taking off on a whim after dinner, and spending more time sailing than futzing with boat bits, I'd suggest choosing the smallest boat you can stand to be on for the time periods you expect to be on it. We spent ten days camping and sailing on a SJ21 Mk1. I would not want to do that trip on a smaller boat. It was primitive. But for hooking up to our small cars, zipping 2 hours to a mountain lake, and launching on dodgy ramps, it's terrific.
You ask what MacGregor 'gave up' to build such a lightweight boat at 26'. A little bit here, a little bit there. Spars, rigging, deck hardware, hull thickness, interior fitout.... Some people might add durability, sailing qualities, and motion comfort to that list, but Macs outsell damn near every other sailboat, so Roger knows his customers. Macs are a solution for owners who want lots of cabin space in a lightweight boat and aren't too hung up on aesthetics or pure sailing qualities. The boats' construction is described as "generally underbuilt but just adequate for their intended purpose." Primarily the Mac26 WB models leave their ballast at the ramp, so that is how they trailer so light. Tradeoff is complexity, performance, and the horror of deliberately introducing water into your boat.
Which freaks me out. Though I suppose our keel trunk is just as bad.