Yeah, exploits will always be a problem with my gaming group. I have started making a list of potential spells that could be problematic for my table but honestly? The problem isn't that anyone can use scrolls...the problem is letting the party have as many scrolls as they want, for any spell they want. That would be problematic even without my "everyone can use scrolls' house rule.
So I intend to avoid this issue the same way I always have: scarcity and time. Shops that sell magical scrolls will have a randomly-generated stock in limited quantities. Crafting custom magic items is a slow and expensive process, per the Downtime rules in "Xanathar's Guide to Everything." Buying a specific scroll is a slow and arduous process, per the Downtime rules for Buying a Magic Item (also in XGtE). As
@Kobold Stew said earlier:
As for
find greater steed specifically? I think things will be fine. If I go with the higher spell DC (10 + double the spell level), and if I keep it strictly as an Intelligence (Arcana) check, it won't be a problem. It's a 4th level spell, so passing a DC 18 Intelligence (Arcana) check would be a challenge for most characters at my table--my players usually tank Intelligence. And a spell mishap with a summoned creature has some excellent potential for chaos.
"The griffon appears! But something is wrong...it's looking at you with hungry, angry eyes..."