It's difficult to say, because the spell Flame Blade is quite underwhelming without OA or bonus action attacks.
I dunno that I'd agree with that - its main competitors at its level are
flaming sphere,
moonbeam, and
heat metal. The last isn't bad, but it's pretty situational. The first is OK, but it's not party friendly. The middle is perhaps the closest direct competitor, and has the edge on damage, but the two spells serve two pretty distinct purposes -
moonbeam for a cluster of weak enemies,
flame blade for the single powerful enemy.
(Also worth noting that 3d6 is pretty amazing on a crit, but Moonbeam can't crit, though this doesn't affect most castings of the spell).
Nah, be honest: it's a cop-out.
The rules say: Hey DM, we could have thought about this, but instead, we're going to make you spend some time thinking about it -- and we'll make you do that thinking while you're busy, in the middle of trying to run a game.
This is not an issue with a lot of campaign-specific flavor. It's pure mechanics.
It is an issue which will have a predictable mechanical impact on several spells, and on some PC archetypes (melee-magic types).
While I think some instances of "DM judgement call" are pretty cop-out (like the stealth rules), I don't agree with that take for this particular spell. The reading of most spells is that they tell you explicitly what can be done with the spell, and anything they don't specifically allow is in the realm of DM judgement.
Flame blade doesn't specifically light things on fire, but I'd probably let it do that, and another DM wouldn't, and we'd both be OK.
Flame blade doesn't explicitly let you make an OA with it, but some DMs will allow it, and others won't, and it's broadly fine either way. The game's not so fragile that this would break it.
This falls within the purview of the DM to shape their own game. If your character build is reliant on making OA's with
flame blade to contribute, I'd say that's a good thing to actually ask your DM about, given that D&D is made to be played by a group of human beings who can talk to each other and not text parsers running on auto-pilot. Discussing what you want to have happen with your character, and DMs making rulings, is part and parcel of
playing D&D, to me. I don't need a super-clinical
Attack line that is only able to be interpreted one way - that's not adding value to the game for me.