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DNS load balancing works good enough if you have smart enough clients (not web browsers), and your pool of server IPs is fairly static. If you can select randomly from a list of names, and then try several of the A/AAAA records from that result, then you may have some delay if you pull a dead server from a cached record, but it won't be too bad. SRV records and really smart clients should work pretty well too, but not a lot of people have really smart clients.

The vast majority of ISP caches won't keep your low TTL records in cache for years, but some do; this is a problem if you have to move your load balancers ever too though.

Depends on how stable your servers are vs your load balancers, and how many connections you need; and if you have enough IP addresses to give public IPs to your servers. Also, if you really absolutely need to control the load precisely, DNS isn't going to ever give you that.