I seem to remember the Romulans' concession was to limit the speeds of their warp drives. Can't remember where I heard it. But I do remember in "Tin Man," the Romulan warbird crew had to tear out their engine safeties just to keep up with the Enterprise-D on the way to Gomtuu.
All true, but invisibility is still a form/type of Cloaking Technology (as I said previously) AND one that doesn't break the Treaty of Algeron. So yes, the Federation has the ability to cloak objects, but it's the Grade-School version whereas the Romulans (and I guess Klingons) have the Masters and Doctorate Level version of cloaks that eliminate ALL emissions and telltales you can imagine. (That is until the Federation comes along and figures out another way to pick up on them.) I think the term "Cloak" is rather all encompassing really, Regardless of semantics if something negates an active/passive scan, then that's object is being cloaked from detection. Whether it's a Ships Cloaking Device or a Land Based Holographic construct that blends seamlessly into a natural formation that is tuned to appear inert on active sensors. An Invisibility suit for Pre-Warp Contact Situations, or Stealth tech to block active scans. They all preform the task of Cloaking an object, entrance, person, or emission in the manner intended, they just go about it in different ways and with varying levels of success.
If you want a detailed explanation of the events leading to the treaty of Algernon and why the Federation agreed to it the read the TLE book "Serpents among the Ruins". The Real-World explanation is, of course, than Gene Roddenberry believed that "good guys" don't sneak around using a cloak.
That's the Treaty of Algeron. The Treaty of Algernon was the agreement not to perform intelligence experiments on lab mice. (Clearly Acme Labs was not a signatory. What do you want to do tonight, Brain?) More likely, the real-world explanation is that an invisible lead starship isn't a good idea in a visual medium like television.
Something occurred to me. If the shady Starfleet officers under then-Captain Pressman on the U.S.S. Pegasus apparently independently developed a semi-operable phase cloaking device, then is it too much of a leap to assume that regular Starfleet vessels in the 2370s/2380s would have little to no trouble installing standard cloaking devices (assuming that the materials aren't rare)? After all, in the post-Destiny novels the Romulan Star Empire gives its allies standard cloaking devices which work fine.
Sure, you could install them, but as I said, there's a fundamental diminishing-returns problem when it comes to power generation: the more power a ship generates, the harder it is to cloak. The kinds of ships that cloaks work best on are stripped-down battleships with no luxuries or non-military functions. And Starfleet rarely uses ships like that.
Possible, but an alternate theory might be that the Treaty prohibits mounting cloaks on ships with capital ship class weapons. Putting a cloak on a duckblind or even using one to hide an unarmed transport ship probably doesn't concern the Romulans, but cloaking a ship that poses a threat to their ships or worlds certainly would.
^As I said, there's a lot more to a ship cloaking device than the kind of invisibility you'd need for an anthropological duck blind or isolation suit. For the latter, you just have to block visible light (in whatever spectrum is visible to the species you're observing). For the former, the important part is blocking everything else -- since hiding in visible light is the easy part. So they are not the same technology at all. They're broadly the same category of technology -- ways to hide things -- but they're quite distinct ways of doing so.
A nadion pulse is part of the operation of a phaser. So a nadion pulse cannon would just be a phaser variant.
If you're thinking of Tezwa, it wasn't the weapons themselves that were the issue, it was where they were placed, and the manner in which they were placed, and who the Federation shared its weaponry with.