Guard basically has all the problems of Rototurret being a low strength sentry without the very useful Trash 1 Program subroutine.

Although Guard will stop Inside Job you still can't reliably rush an early Agenda behind it because Self-modifying Code, any Killer or an AI breaker will eat it for breakfast.

The worst part about Guard's 'anti-bypass' ability (which also 'stops' Femme Fatale, Feint or Security Nexus) is that it doesn't stop the most common type of runner bypass! I'm talking about Blackmail, DDoS and Leela Patel: Trained Pragmatist bounce.

Guard is over-priced and under powered. Every faction except have better Sentry Ice that can End the run. and Jinteki would rather use their sentry ice to end the runner.

Rainbow can be broken by any Icebreaker in the game except for Mimic, Yog.0, D4v1d (technically not an icebreaker) or Grappling Hook (also technically not an icebreaker).

Mimic and Yog.0 can easily get through Rainbow with the help of their friends Datasucker and Net-Ready Eyes.

I suppose Gingerbread, Deus X and Sharpshooter also count but they will often be accompanised by Panchatantra.

Quetzal: Free Spirit can walk right through (once per turn). Depending on the positioning Rielle "Kit" Peddler: Transhuman might be quite happy too.

With a weaknesses like this Rainbow would need some major upsides or strengths to warrant inclusion in anyone's deck.

1. 3 to rez for a Str 4 'hard' End the run.
2. As the outmost piece of Ice on a server it will 'take the bullet' from Forked, Knifed or Spooned, protecting your better ice from destruction.
3. Str 4 also means it either takes 3/4 turns or a number of Datasucker counters to trash with Parasite.
4. Against decks using an AI breaker like Eater or Faust it is no weaker than a Bastion.
5. Zero influence.

So we can say that Rainbow is kind of 'strong' against Ice Destruction and okay against AI breakers. But if you don't like Ice destruction you've got Architect and Lotus Field. If you don't like AI breakers you could use Swordsman, Turing or Wraparound.

Does it combo with anything?

1. Superior Cyberwalls counts it as a barrier
2. Encrypted Portals counts it as a code gate
3. Unorthodox Predictions makes it unbreakable for a turn

Not an impressive selection in terms of combos either.

The only circumstances where Rainbow will outperform other ice is if your local meta is full of Anarchists who use AI breakers and ice destruction but don't have a lot of Mimic/Yog.0/Datasucker. If this is the case, go ahead and make them taste the Rainbow. Otherwise it's hard to have much pride in this piece of Ice.

A not particularly impactful piece of trap Ice for any corps apart from Chronos Protocol: Selective Mind-mapping, which will get good value by trashing a card of choice from the runner's hand. While it is free to rez it does take up a card slot and a to play + installation costs. Analysing the economic side of Data Mine we see the corp invests a card and installation costs in order to either tax a runner with an AI breaker or to take out one random card from the runners hand. Unless the corp was lucky to hit a particularly valuable card, the runner has only lost a to replace the card. It is unlikely that Data Mine will lead to a flatline outside of Jinteki: Personal Evolution and that is only if the runner is taking dangerous risks.

This card sits in a faction that already has a wide selection of high quality cheap Ice such as Pup, Yagura, Cortex Lock, Himitsu-Bako, Lockdown and possibly Harvester or Chum. Often in you will not have a lot of spare slots (especially not Ice slots) in your deck so this Ice will not see much play outside of Chronos Protocol.

Is this card nothing but a problem looking for a solution or is it actually a useful strategic tool?

Lets look at what corps typically spend doing:

1. Melange Mining Corp. - Preventing this is annoying, but won't cause them to shed any tears.

2. Purging Viruses - Now we are talking - lets say you've Parasited a big piece of Ice and they've let it tick down to 1 strength, thinking they would purge next turn, what about if you have built up a huge Medium and want to keep hammering them next turn. Even to keep a pair of Imps locked and loaded to wreck their HQ next turn. You can do a bit of damage with this card, assuming they don't have a Cyberdex Virus Suite on hand. What kind of seedy cards can you use to trigger Populist Rally? How about Wyldside, Scrubber or Street Peddler? Sadly Aesop's Pawnshop is too classy to be involved in this populist hoo-ha.

3. Triple Advancing a 3/2 Agenda they just installed in their scoring server. Another interesting situation where you might wish to delay them one turn until you can make your big play for the scoring remote. However Populist Rally will set you back 2, a card and a to delay them one turn. Keep in mind that this is wasted resources if they only installed a Caprice Nisei or Adonis Campaign in that server.

So it looks like there isn't that much that you can do to mess up the corp's plans with Populist Rally. Is it useless?

1. Fisk Investment Seminar then put some more cards in their hand with Leela Patel: Trained Pragmatist or Laramy Fisk: Savvy Investor. Now cut them down to only having on thier turn to deal with being 5 cards over their maximum hand size. They are going to be praying for Jackson Howard. This would be a good time to activate an Eden Shard too.

2. Speaking of the corp's maximum hand size, how about Valencia Estevez: The Angel of Cayambe, Investigative Journalism/Activist Support etc. and Itinerant Protesters? For even more fun you could combine this with Fisk Investment Seminar to really ruin their day.

EDIT for Salsette Island: Taking away a will also prevent the corp from using Jeeves in a lot of cases.

Prey has an incredibly useful effect and is far more general than Forked, Knifed Spooned and Run Amok which all have restrictions and conditions regarding which Ice you can target. Unfortunately Prey asks you to trash installed cards, which is typically undesirable outside of Apex: Invasive Predator or maybe Noise: Hacker Extraordinaire decks. It is already clear that Datasucker or Ice Carver will not help you lower the strength of Ice for Prey and using Scrubbed could be considered a little excessive. So how do we lower the strength of Ice outside of an encounter?

Bishop is the only option apart from Parasite to lower strength outside of runs.

Wyrm: One of the most derided and disliked AI breakers has finally found a purpose. It is time for this ugly duckling to find a place in the sun. Since Apex will already have e3 Feedback Implants as well it might even potentially be used as a backup breaker, although Wyrm is still hideously expensive for that purpose. Maybe a backup breaker for the backup breaker? You will need an extra 1 for that however.