They could be right, but we have been doing it here for years with no ill effects.
The answer likely depends on the size of your installation and your hardware. We have one incoming PRI, and our system handles Trixbox 2.2.4, Hylafax + IAXModem, Queuemetrics, and all of our call recording (all inbound queue calls) without ny problems. This is on a Pentium 4.
Your mileage may vary. If you could get details as to what 'known flaws' they are mentioning, maybe others could confirm that.
Tom