The disadvantages are you must leave every 90 days and every 2 months you pay 1900 to extend. It is far cheaper to get a soft power DTV. It costs 10,000 baht for 5 years, it's multi-entry, can stay 6 months, no need for extensions (although you can extend once a year), minimal dealing with immigration, and no need for money in Thai bank (once it is issued there is no requirement for money in any bank).
The trouble with bank staff in Thailand is that they do not understand the visas, and why should they. Her boss has probably misunderstood instructions from head office. At one branch I was told I couldn't open a bank account with a non O because it wasn't a visa. If I get an extension then I can open a bank account as that is a real visa. When I informed her that you cannot get an extension without a bank account she just shrugged her shoulders. I have a BBL account and when I asked to open a second account they told me I need a yellow book, work permit or letter from my embassy. I asked what type of letter from the embassy and she didn't know. The whole thing is a farce
Before your next visit to Thailand obtain a Non Immigration O visa based on visiting Thai family. With this visa you can open a Thai bank account and also visit the immigration office and ask what is required for an extension based on marriage. They will give you a printed list so you can start to get your head around the requirements. Meanwhile start saving for your 400k baht deposit.