Dash isn't a currency, it's masternodes take coins out of the markets. It's kind of a 2 way loss, if the masternode is up and running then at least 1,000 dash is out of the market and locked on the masternode. If the masternode isnt running then thats 1 less node thats "securing" Dash's "anonymity" and makes it more vulnerable to attack(Less amount of nodes to take down). Also, there are currently 2million Dashes locked up in masternodes, making darkcoin an extremely volatile cryptocoin, which is why it's so easy to buy/sell and the price goes up/down immidiately, meaning Dash has relatively bad liquidity and cannot be used a currency. That means since Dash can't be used as a currency, it's only other options left is being used a commodity/speculative investment, which it is and can't ever change from being.
Couple the extreme lack of liquidity from over 2milion entire Dashes themselves being tied up in masternodes, to the initial instamine where another 2million were mined by very few people, and that means Dash is an extremely Illiquid cryptocoin. So regular users wont be able to buy Dashes unless they like moving the price a lot everytime they buy/sell. Also, some people would say(That's what denominations is for), well that would be true If there was no masternode locking up 1,000 Dashes per node and if there wasn't a instamine where 2million dashes were mined in less than 2 days. Even if you use a smaller unit of account, there would be a substantial amount of coins "missing" and people generally do not like using decimal systems, especially for a cryptocoin that has a relatively low price(Bitcoin excluded since it's price is relatively "high" and denominations are actually useful in that situation, not for any other cryptocoin though).
Next, Dash isn't a "legitimate" coin. The instamine happened where the block reward and coin supply were drastically changed, it doesn't matter what happened with the coins, it doesn't matter if the original instaminers are holding all their instamined coins or if they sold already. The point is that it happened, where things that are never supposed to be changed, were completely defecated on. That destroys the entire purpose of being a cryptocurrency, where its "parameters" as some call it, should never be changed, lest you create huge inequality. Dash's "parameters" being changed so drastically shows that the coin was never decentralized, and honestly can't be taken seriously. It's pretty scam-like that it happened.
The nature of Bitcoin is such that once version 0.1 was released, the core design was set in stone for the rest of its lifetime.
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Then there's the masternode and how masternodes themselves bring up lots more vectors of attack, dangers of centralization, spying, etc etc. Too many flaws for me to type without writing out a 5 page essay.
There is no liquidity issue with a currency that is divisible to 8 decimal places!
look at this weeks volume on the exchanges, we have seen 100BTC buy orders stacking all over the place with steady increases everyday, if someone try to buy 100BTC of XMR it would send monero up by 1000%...
Lastly check out Kristov Altas's excellent review of Darksend and the mastenode network none of your concerns are an issue.
There are no attack vectors with masternodes , they are completely blind to transactions.