Can you define terrible in this context and can you provide information that is current and relative from the past decade about these filters being spoken of with respect to their actual wavefront?
I'll describe in a bit more detail what I did and saw >>>
We were qualifying laser-attack shields. They look just like clear glass windows, but are coated/tuned to reject 99+++% of offensive radiation. We astronomers think of a couple dozen layer coating as extravagant... but these are hundreds or more nearly magical concoctions. No surprise that beyond superlative coatings merit beyond superlative substrates... even for little few-inch witness-sample windows. We checked these in double-pass interferometry. And gosh --- etalon quality with even zero wedge. A relatively thin window that you can hand hold insert into a fluffed double-pass cavity and still see perfectly fluffed cavity (cept for the piston) is quite astonishing!
So I took advantage of our set-up and we guys started sticking every optical window we could find in there just to educate and amuse ourselves. This included my tubs of commercial astronomy filters from home, camera filters, camera windows, solar filters (with the coatings stripped off), plain old window glass, float glass, optical pellicles, thin Mylar sheet, Saran Wrap, unblown soap bubbles, old-fashioned polished plate glass, WWII optical windows, 6" and 10" thick fused silica optical windows from Edmund, etc., etc.
What I found was that (as expected) etalons are very good and our custom Edmund windows also very good (1/10-1/20 wave single-pass wavefront PV). Ditto the optical pellicles, some of the hand-crafted WWII parts, and old-fashioned polished plate... which is no longer produced. Virtually everything else (that we tested) was closer to window glass quality than ~optical quality~. But this is also not surprising, and why most filters cost ~$50 typ. Otherwise, the glass substrate alone would eat up the entire cost.
Terrible = few waves typ PV (random/irregular) SP over a couple inches diameter. And that will generally be good enough for filters residing reasonably close to focus. My recollection is that Premium Camera manufacturers offer fused silica optical quality integral camera windows as an option. FLI comes to mind.
If Baader filter substrate is 1/10λ --- That's Superior! Tom