I have no personal experience to offer, but I've heard nothing but bad things about these systems. The fact that such equipment is provided by small (relatively unknown) equipment companies and not the big guys is convincing enough for me.

I know of two alternatives, there may be more:

1) A standard system using ducts covered with bulkheads/fur downs/chases/ choose your word. This is used in new construction, no reason to shy away with a retrofit.

2) Install a mini-split (ductless) system.

I have installed Unico systems in a number of houses with hydronic heat.

It is a different design world than typical forced air, but not really any harder to design.

It does take some creativity to get the main lines in as needed, and duct-board works well in unconditioned space (lower thermal mass to load the system).

If you can find an experienced designer/installer it is not all that hard.

I installed Spacepak rather than Unico in the course of renovating a 1923 Sears kit house that has radiators. After a couple of bumps early on, the system works very well.

If I had it to do again on an older home, however, I think I would have tried to find an installer certified by the manufacturer of the system.

Best of luck.