I'm sure a lot of dive computers have a similar disclaimer that it's not intended for DECO diving. That just protects them from some level of liability because like previously stated, the computer doesn't know how much air you have left.
However at the same time, the computer manufacturer knows people are going to want DECO level functionality. I've only DECO dive'd twice, the deeper of the dives (110'), I entered DECO at around 77' (since I started at the bottom of the wreck and slowly worked towards the top) and after a few minutes began slowly ascending and arrived at 55' to do a 5 min DECO stop, at this point I still had 1,500 PSI left. Before the dive, I had not planned on going out of NDL, but since it was a wreck I told myself I might. I pre-determined with my buddy that regardless what the computer said, we were going to begin our slow ascent with no less than 1,300 PSI. We both felt that it gave us sufficient air to complete 10-15 minutes of DECO/safety stops. Regardless of what the computer told us, we had also pre-determined regardless of NDL or DECO that we were going to stop at 55' for 5 minutes (roughly half the max depth). After that my computer instructed on a 2 minute deco at 25', we did that, then we did a 5 min safety stop at 15', and surfaced. We still had around 900 PSI left when we surfaced (both my g/f and I are above average when it comes to air consumption; meaning we use less than others, especially when not moving or at safety stops).
That said, air time is a critical factor for when doing deep and/or DECO dives. If I'm diving past 90' then my buddy and I will always have a pre-determined plan regarding predetermined safety stops and air time limits with a high level of conservatism built-in.
I'm sure some people will feel my system is flawed so if someone has any constructive input feel free, but I verified my system with a very experienced technical diver and he thinks it is plenty conservative.
