Is there a difference in using decat (sorry , do not have Romanian keyboard) or ca in the comparative form or are they interchangeable ?
When you say you are like something, I mean equal to something you use "ca", but when you're saying that you're better or worse than something, you use "decat". Sunt la fel de bun ca Andrei. ( I'm as good as Andrei) Sunt mai bun decat Andrei. (I'm better than Andrei)
Indeed, "decat" does not work for two things that are equal/alike - it may be easier to remember that in comparisons "Decat" reflects a Difference. However, "ca" is pretty much universal in this context, it should be fine for better/equal/worse in relatively simple constructions. Things may get more complicated when the second term of the comparison is another sentence/verb, but saying something like "Sunt mai bun ca Andrei" would not be wrong. (Romanian is my native language.)
Note: avoid using "ca" followed by words that also start with "c-", especially "ca-": cafeaua, camasa, caietul, etc. - it would create a cacophony.
I'm not quite getting why in the example linked in the first comment Duolingo would possibly dislike the use of "ca". I almost wonder if in fact it wants the word "kitchen" in "kitchen stove" translated too.
From https://dexonline.ro/definitie/ca : "Intră în structura gradului comparativ al adjectivelor și adverbelor" (It is part of the structure of the comparative degree for adjectives and adverbs), e.g., "Mai înalt ca tine?" (Taller than you?)