That's not what I hear from reputable reverse engineers, at least for IoT devices.
Even though security by obscurity should be frowned upon, and being understood that hiding code might give a false sense of security, most RE workflows assume availability of firmware, and it is a giant pain to start breaking platforms where the unknown firmware must be manually extracted first, especially the boot loader.
I've said before on this site, obscurity is a totally valid tactic to impose additional costs on attackers. It's best to think of it more as a preemptive strike than a defensive layer. Thinking of it as a defensive layer can lead to complacency, but thinking of it as an opening gambit is totally fine.
One shouldn't be obsequious to heuristics like "never use security through obscurity," one should understand the systems they're building and make considered choices.
Additionally, preventing people from dumping your firmware is usually not about security as much as it is preventing some fly-by-night company from reversing your product & selling it as their own. Why engineer a product when you can steal someone else's IP?
If you hire those security engineers, you can give them access to the source code. If you didn't hire them, there is no way to tell benign attackers from other attackers, and they can deal. It probably won't give them much trouble anyway. :)
The better way to deal with this is to make your firmware secure regardless of whether someone can pull it off your device. Making security engineers’ lives difficult just means that you’ll find out about bugs from the news when they’re being sold to authoritarian countries to suppress dissents rather than the paper on firmware security that a graduate student was going to write before they decided to move to a much nicer platform.
The first is that my thesis is explicitly that this is not a defense and is not an excuse for poor memory handling etc. in your firmware. (And the more I invest in creating a robust firmware, the more I stand to lose if someone rips off my product & undercuts me - security risks are not the only type of risk.)
The second is that the notion that I should rely on the charity of unpaid graduate students to discover bugs in my firmware is both inequitable and unsound.
But there's the rub: you'll only impose additional costs to the least sophisticated/determined adversaries. While that works to keep random scriptkiddies/scans out, I'd argue it has little to no effect if you require serious security guarantees.
The reason I say to think of it as a preemptive strike rather than a defense is that you still do need strong defensive layers.
This is basically just setting a compiler flag. It's free for you and costs something for the attacker.
Not a big impediment.
For 9 out of 10 consumer devices, no factory will even listen to you if you are not willing to commit to put forward $1m
Only really simple, small gadgets are profitably manufacturable under <$100k