Ok I know one can use solids on a hydraulic cam but what are the problems of using hydraulic lifters on a solid cam?

The ramp on a solid is different than a hydro, the seat duration would be much more & depending on the cam the valves would not be closed when they should.

In an emergency when I tried that on a street solid roller in the shop truck I ended up with rougher idle (increased overlap?) but it still drove around okay. Never did test it on my dyno so I cannot speak to power curve changes.