Alright Kirby, I'll play your game here. Let's compare these items:
Euls + Force
- Cost:
- Stats:
- 20 int
- 150% mana regen
- 10 dmg
- 10 as
- 30 ms
- Actives:
Manta
- Cost:
- Stats:
- 26 agi
- 10 str
- 10 int
- 15 as
- 10% ms
- Actives:
Summarizing:
- Manta costs 100g less
- Euls + Force gives 10 more int (130 mana)
- Manta gives 10 more str (190 hp)
- Manta gives 26 more agi (16 more damage 2.2 armor)
- Manta gives more AS (31 more from agi + passive)
- As long as you have boots, Manta gives more MS (+4 ms with red boots)
- Euls + Force gives more mana regeneration
Comparing actives is harder. There's no denying that all of the actives are very useful, but you have to consider what hero is using them and their role in the game. I'll get to actives specifically in a bit.
An exaggerated example would be something like a CM getting a radiance, as her low HP and poor base AS disallow her from taking advantage of the item's max potential. Viper has two spells, one of which is an orb with a very low mana cost, so the mana regeneration from Euls as well as the bonus intellect, (130 mana) is largely wasted compared to something like Lina with more actives and more dependence on her spells.
Viper is generally classified as a powerful mid-game carry that can be tanky, and counters auto-attack heroes later into the game. His presence in the game comes from his ultimate and auto attacks. So the additional damage, attack speed, and movement speed for chasing from Manta allow him to fulfill this role more effectively.
Viper's offensive passive, nethertoxin, allows him to do plenty of damage without many damage items, so giving him survivability goes farther with him than most other carries. Combining his defensive passive, 25% as/ms slow with 25% magic res, with more HP compounds the benefit, so the additional 190 HP from Manta is very beneficial, as well as the 2.2 bonus armor.
On to the actives, as I said before this comparison will be a little more difficult as they vary based on team composition. Generally, you don't want a carry/semi carry spending their gold on crowd control items that supports/gankers could be buying. The cyclone gives viper a little more chasing potential, as he could cyclone a fleeing enemy from 700 range rather than attacking and slowing from 600, but the gap in movement speed would probably balance this out while the cyclone is on cooldown. Combining cyclone/push allows for much more chasing potential, but is that really worth the 5000g? You trade off all of the benefits listed above to do a worse job of killing the enemy once you get to them.
Cyclone and push have defensive and in-fight applications as well, but so do illusions. Most spells that could be dodged with Cyclone can be dodged with Manta split as well if used at a high level. Manta arguably leaves you in a more favorable position afterwards as you don't hang in the air and let enemies position themselves and instead come out with illusions. These illusions both do damage, even though they don't use nethertoxin, and serve to confuse opponents for a few seconds while they don't know which illusion is real. Saying that he doesn't benefit from illusions is pretty nuts as he benefits more than most other heroes given his constant slowing ability letting the illusions get more hits in. You lose the ability to save teammates with push or move enemies out of position, but that doesn't matter much to Viper as that isn't his job.
Even if you wanted to get a crowd control item in a dark situation, and these situations do arise, sheep is clearly the better option, as it would allow you to do a lot of damage during the disable compared to the invulnerable cyclone.
What it boils down to is that while both of those items could be used and you could find some success with them, they don't augment Viper's real strengths well enough to be considered anything more than troll items.
"No, how dare you experiment and question conventional builds, you must do what everyone else does because that is has been proven to be the bestest way and no way can be better."
I had hoped my blunt response to the idea would provoke analysis of why I would say something like that and why it's a conventional build, as you seem to love investigating dota arcanery. Instead you just cover a silly idea with the "experiment" excuse.
EDIT: Forgot to talk about Yasha only:
The damage, armor, and HP from the the Manta upgrade, as well as the illusion benefits that are discussed above, makes the upgrade to Manta very worth it in my opinion.