First off it's a game, unlocking one tile gives you an option anywhere from 0 tiles to unlock all the way up to 20-30 tiles to unlock.  Having the choice of which one tile you want to unlock is a great choice to have especially with adjacency bonus's. 

However, as you research further up the tech tree for all but the Altarian's you eventually get to a tech that allows you to unlock as many tiles as you want that meet  certain requirements.    With that you could take a class 8 planet and turn it into a class 36 planet if the planet supports it.

I've been thinking that the terraforming needs to be scaled to reflect the quality of the terraforming.  For example:  Say you start with a Type 7 planet.  The usable tiles versus total tiles averages out to a 7 total planet quality.  You have a couple Type 14 tiles, a few Type 12s,  a few more Type 10s, a LOT of Type 8s (water tiles), but then the rest of the land tiles are <Type 6.  Add them all together and average them = Type 7.  The first tech comes along and that would allow you to improve a land tile to Type 10.  The next tech allows an upgrade to Type 12.  Etc., until you get to Biospheres where even water tiles can be upgraded to Type 12.  The catch to this multiplicity is that once something is built on a tile it locks the tile quality to that level of improvement.  "Gaia" would be a planet of ALL tiles being Type 20+.  But an impatient player would start filling up all of his tiles that were Type 8 or better ASAP.  Deciding to plant a Farm on a Type 8 tile would make it less productive than a Type 14 tile.  Building any kind of Manufacturing structure would reduce a tile's Type.  Morale would be affected by the planet's average Type value, but particularly by the tile quality where Commerce, Entertainment, and population support structures are built.  That in turn affects things like population growth rates, Influence rates, Commerce profitability, etc.

The question becomes, "Build now, or wait until I've raised the terraforming tech more?"  And thereafter, "Is it worth it to destroy a structure to terraform that tile, and then rebuild the same structure?"  ((Like, Urban Renewal.)

I realize hardly any players would want to get into the nitty-gritty of planetary quality calculations.  So it simplifies down to, "Do you want an empire of slum planets?  Or would you prefer an empire of Gardens of Eden?"

I'm making a custom faction that has the capability to mass-terraform right away, but only to the point that they can turn bad planets into decent ones, or at least, that was the goal. Here's what I figured out while trying to balance this:

1) Terraforming eligibility is determined by what % of a hex is taken up by "land". Planet class has no direct effect, though planets with higher starting class usually start with more land, and these worlds also have a higher upper limit than naturally low-class planets.

2) Some planets have no land to improve, both high and low, so while this does mean you might get a class-22 that's as good as it's going to get, there are also planets so bad that there's simply no way to make them better than "Earth-Like", even with an Ultra-Terraformer. The custom faction star system with only one Dead planet as a starter is a good example.

3) Some planets start as high-class, but have a huge amount of empty land, so they can be improved greatly even with really crappy mass-terraforming technology. The starting planets of the Iconians and Iridium are extremely good examples. They have HUGE, empty, Pangaea-like continents, and if you have any kind of mass terraforming, you can jack them up to class 30, because they are loaded with hexes that have 100% land. It's even stronger than it sounds, because those tiles are also in nice neat groups that let you stack adjacency to the moon.

4) Mass Terraforming is EXTREMELY GAME-BREAKINGLY POWERFUL, even if you make the upper limits REALLY bad compared to single-tile terraforming. I initially had two levels of mass-terraforming in my custom faction, both much worse than the Ultra Terraformer, but available earlier. My custom faction would get the first level, then take off like a rocket and squash the entire galaxy beneath its heel, all before even getting to level 2. Even after many nerfs, that mass terraforming remained so unbelievably overpowered that I had to cut the second level out entirely and heavily nerf level 1, plus consider locking it behind Age of War.

You know the Ultra Terraformer that everyone gets at the end of the game? It has the same "power level" as the very first Soil Enhancement you get, except it can be built until there's no land left to improve. Even with my custom faction's best terraforming option only half as powerful as that, they could snowball out of control extremely fast, because every planet they colonized would end up class 22 or more.

I imagine the devs reached the same conclusion that I did: if you want there to be mass terraforming, you can't have multiple levels of it, because if you make one of the levels too weak, you can barely use it at all, so you can get utterly screwed by the RNG, and it's not fun to use, but if you make it even one percent too good, which is extremely easy to do because each percent increase makes it exponentially more powerful, anyone who doesn't have it is as good as roadkill to the guy who gets it first. It becomes an over-centralizing tactic.

So, when people say they'd have to re-design the entire economic system to give people mass terraforming, even weak mass terraforming, they aren't kidding.

That might be a good idea for a TC mod. However, despite common interpretations stating otherwise, not all terraforming involves re-forming the entire planet, and one could argue that the Ultra Terraformer IS exactly that, while the others are just incremental measures that, say, purify the soil or build a dome/air scrubber that makes desolate patches of the planet habitable. If you Terraformed the Gobi Desert into a grassland, for example, that might be what Soil Enhancement is (you can totally do that in-game, too). You might even say that's what we did with Los Angeles. Using Los Angeles as an example again, we did that by taking water from one place, and moving it elsewhere, and since that's basically what low tier terraforming is, that is why you can't build it en masse (you have to draw from the planet's own resources, which are finite). The fact that large-scale terraforming is an endgame tech just goes to punctuate how difficult it is, where even in the magical land of FTL and easy Fusion power, terraforming an entire planet remains a massive undertaking. The Ultra Terraformer tiles are EXPENSIVE to build, totaling even more than your proposed mega project, so it is adequately priced for how powerful it is.