I can't really see that this is a question that can be approached technically (and perhaps not even objectively), it seems more a matter of taste and culture, but I will attempt to address your question as best I can. I must say, though, I find it a little hard to follow you.
Let me begin by saying I personally really like the abugida approach and have always found it makes a lot of sense. When I invented a script of my own once, it was an abugida. But they're not objectively easy to work with!
1-1 correspondence between letters and syllables, upto 100% consistent.
I'm not really sure what you're getting at here. It's true that abugidas are built around the syllable and a single glyph in an abugida represents a syllable. But there's only a 1-1 correspondence between glyphs and open syllables. Closed syllables (syllables that end in a consonant, like English 'cat' or Hindi रात rāt) need two glyphs for one syllable.
Furthermore, although by convention something like को ko is treated as one glyph, it's clearly made up of two parts! Is that better or worse than 'ko'?
It's also not clear why a 1-1 correspondence between glyphs and syllables should be preferred to a 1-1 correspondence between glyphs and the phonemes that make up syllables. Alphabets are designed to represent the individual phonemes rather than the whole syllable. Is that a disadvantage? Not objectively, I think. It's certainly a lot less hassle as you don't need a different glyph for a vowel at the start of a syllable to a vowel after a consonant!
Now it's true that the English alphabet does not have a 1-1 correspondence between glyphs and phonemes, but that's a fact of English writing and not a necessary truth of alphabets in general. It's possible to design an alphabet with a 1-1 correspondence between glyphs and phonemes. It's also possible to have an abugida which does not have 1-1 correspondence between glyphs and syllables, and we see this in Hindi where त could be ta (as in तब tab) or t (as in रात rāt).
Combinations of consonants and vowels enable learning only a small number of symbols
Forgive me but combinations of consonants and vowels requires you to learn both the independent vowel (e.g. इ i) and the dependent vowel (in Hindi this is called the vowel mātra) (e.g. ति ti) and any special cases, such as रु ru and रू rū. And that's before we've even got to the conjuncts you mention in your next point. In an alphabet, an 'a' is an 'a'. You only have to learn it once. (It's true that in English we have upper and lower case, but case distinctions are not a necessary characteristic of alphabets.)
Conjuncts allow unlimited combinations of consonant sounds (theoretically ; some sound just awful)
Again I'm genuinely confused. In an alphabetic writing system you can arrange letters however you like to represent any combination of consonant sounds. With abugidas, well, I invite you to look up any list of Sanskrit conjuncts and you will see that it can get very difficult to join increasing numbers of letters. Few fonts can handle the relatively common Sanskrit conjuncts dbhya and ḍbhya for example. And this is actually very relevant, as you will soon see, if you look at lists of Sanskrit consonants, that a lot of effort has gone into formulating rules for joining characters together in an elegant and pleasing way. Which means familiarising yourself with conjuncts can mean learning dozens and dozens of extra glyphs individually! In an alphabet, I can write 'dbhya', 'dhbya', 'bydha', 'dlqwdyddda'. I can write whatever I want without changing the letters at all.
What are the disadvantages of Abugida writing systems?
Complexity, as I hope I have shown. They also don't work nearly as well for languages they weren't designed for. For example, Tamil is an abugida, and the script is now commonly employed in the Tamil country for writing Sanskrit. But Tamil has only one t-letter (த்). Whereas Sanskrit has t, th, d and dh. Now there are ways around this, in the same way that in English I need to use diacritics (such as 'ā') to represent foreign sounds, but these ways do rather destroy that elegant 1-1 correspondence between letters and syllables. With alphabets we can put letters in whatever combinations we want really.
Or is it odd that any writing systems other than Abugida and Alphabetic exist at all?
Not really, as I say, it seems to be a matter of culture. As you clearly understand, syllabaries work like abugidas, but each syllable has its own individual glyph, there aren't regular patterns. It's true that this requires more hard memorisation and it makes these writing systems poorly-suited to representing foreign words, but they work well for the languages that use them.
Pictograms/ideograms/logograms do take a lot of work to learn and are very complex but it's easy to see how they developed and of course the original pictographic writing would have been far easier for early humans to develop and understand than abstract symbols with complicated rules.
Abjads sacrifice absolute precision for, I suppose we could say, 'compression'. Shorthands tend to be abjads because you can make do without the vowels.
