Can't comment on Shinobido 2 since sadly it was Vita only, but since you said series, I can give a run down on the first game.
For me, the first game was superior to Tenchu, if you can imagine blending Way of the Samurai's staple features with Tenchu's ninja formula and you basically get Shinobido. To progress the story you have to do a certain amount of missions for one of three factions, only each of these factions has sort of an approval bar toward the player - do more missions for a faction, they'll like you more, offer more money, better jobs, easier access to their homebase, etc. On the flipside, if you're witnessed working against a given lord, that lord isn't going to be too happy with you and there will be consequences in the form of reprisals and refusal to offer you missions. The ultimate goal of the game is to have one of these factions take power, although without spoling anything, I'll simply say there are other possible endings available.
For a given mission (which come in many varities), in addition to the enemy faction that you'll be working against, there *may* be a enemy ninja faction (1 of 3) that is hired on to help protect your enemies (this only happens after a certain story event and is meant to mix things up by way of random modifiers) - this means more, tougher enemies, who will call for help and will likewise bear grudges. To further mix it up, the ninja faction leaders may or may not be present on a given mission and their presence can seriously impede your plans. If anyone spots you on a mission and lives to tell the tale, that faction's lord will hear about it and will obviously distrust you. Keep on down that road and they may attack your hideout and stop offering you the better missions, so managing witnesses is important. In essence you get a sort of emergent narrative quality to the game that runs concurrent with the main thread, it's pretty neat, especially given that if you're skilled enough (don't get spotted) you can work against a faction and they'll never know it was you, this can lead to some really satisfying double crossing and let's say may lead to interesting story developments.
Gameplay and mechanics wise, again it's superior to Tenchu, the running felt more fluid and ninja-like, the levels are more open giving your grapple genuine use. The combat and assassinations are satisfying, even moreso as new enemy types get added to the mix and you have to adapt your strategy prior to and during a mission. Imagine your mission is to intercept say a convoy of grain and the proverbial fly in the ointment is the opposing faction has hired a ninja faction to stop you. There's nothing more satisfying that dropping some mines and bombs along the faction's route to tip the odds in your favour when it's time to strike. On that subject, you have a neat alchemy system that you can exploit at your hideout to make your own custom potions and bombs (it can kind of be broken in a similar way to say Oblivion's custom magic, but that's half the fun tbh) that can elicit a variety of effects in enemies. Oh yeah, your hideout is customisable too, so you can essentially rig the surrounding environment with traps (if you've ever played any of the Deception games, it has a passing similarity to that I suppose).
If you've played Way of the Samurai, Shinobido can be likened to being the ninja version of that, same developer and all. Like WotS, it's criminally unknown and underrated and its very difficult to succinctly capture everything that makes it such a great ninja game, so my best advice is to try it, there's a lot more depth and more going on in general in Shinobido compared to Tenchu that defies direct comparison imo, but Shinobido is a game I returned to many times over, the Tenchu's not so much.
